There is a workaround, however, if you want to keep your head-unit as BMW intended, which isn't perfect from a sound-quality point of view, but it works great none the less. A left and right RCA channel can be wired from the rear-speakers and the remote set to come on with the ignition, all from the boot so wiring can be kept to a minimum.
** The amp feed from the rear-speakers means there's no separate controls for the amp on the head-unit. The RCA will receive an unfiltered audio-signal set at the main volume, so an amp with onboard volume control and lo-pass filter is recommendable if you want a decent sound similar to a head-unit RCA setup. The gain of the subwoofers can be adjusted using Bass +/- on the head-unit Tone control, but the bass will likely still be out of balance with the cabin-speakers.
The entire process is quite involved so I've split the guide into sections, linked below:
Wiring Power / Ground to the Amp
Removing the Rear Seat / Parcel Shelf
Wiring RCA from Rear-Speakers
Getting a Remote Signal for the Amp
Adding an On/Off Switch
WIRING POWER/GROUND TO THE AMP:
The battery being in the boot on E39s makes it very easy to power an amp, but the red Power cable supplied with most wiring-kits is about 5m long so it might be worth cutting it shorter after the fuse-holder to save hiding it all away.
1. Fasten the ring end of the red Power cable to the auxiliary bolt on the battery live-terminal using a 13mm wrench.
2. Fasten the ring end of the black Gnd. lead to the car's main earth-stud beneath the rear light, using a 13mm wrench.
3. Run the cables through the aperture in the boot-lining with the other power cables and out of the top by the suspension-turret, so the wire doesn't exit around the panel door/hinge.
** If you're mounting the amp centrally on the boot floor it might be worth running the cables under the rear-half of the boot floor and drilling holes for them to come up to the amp through, although this will probably mess up the folding boot-floor.
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This gets quite involved, but its the only way to access the rear-speaker wires and get an audio signal for the RCA. If you're using an after-market head-unit, then you'll still need the back seats out to run the RCA/remote cables under the carpet/sill and into the boot.
1. Pull the front edge of the seat squab upwards until it pops out of its base, then pull it forward to remove it.
2. Pull the centre-headrest upwards until it pops out of its mount.
3. Remove the two screws holding the seat-back in place, located on the plastic-trim at each bottom corner, using a 10mm wrench.
4. Lift the base of the seat-back upwards a couple of inches until it is clear of its 2 lower and 2 upper brackets and can be removed.
5. Remove the centre-headrest mount by undoing the 4 bolts using a 2.4mm allen-key.
6. Remove the 4 push-studs from the leading-edge of the parcel shelf by pulling out the centre-pin, then the stud.
7. Lift the leading-edge of the parcel shelf until you can reach under to the headrest tube-brackets, then use a screwdriver to slide the wire-clip off the recess in each tube. Lift the headrests out of their brackets.
8. Making sure the seat-belts are not trapped, slide the parcel-shelf forwards out from under the C-pillar trim and rest the shelf against the seat-back. You may need to carefully free the edges of the shelf from the trim with a flat-screwdriver.
Re-fitting is a reversal of this process. Replace the 4 wire-clips to the headrest tube-brackets, with the straight side of the clip across the recess in the front, before replacing the parcel-shelf.
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WIRING AN RCA CABLE FROM THE REAR-SPEAKERS:
The amp gets its audio signal through a normal RCA-cable from the head-unit, but BMW Business stereos without a standard amp don't have one, so the best place to take a signal from is the rear-speaker wires. This means no separate controls for the amp and its an unfiltered pre-amplified signal so an amp with a volume control and lo-pass filter is recommended. Cutting the ends off a pair of stereo RCA cables is the quickest way to get a feed from the speakers, making use of the existing neat wires and plugs. Co-axial cables are fiddly to cut and splice though, so 4 lengths of speaker-cable or thick wire will do and RCA plugs can be got from places like Maplin for about 50p, or nicer gold-plated ones online for £3 and up.
1. Cut the plugs off one end of a stereo RCA-cable and strip the wires about 2" from the end, exposing the large bare (-) wire and the small red/white insulated (+) wire.
2. Separate the two wires and twist the large bare wire together, bending it away to keep it as far from the red/white wire as possible.
3. Carefully strip about 1cm of the insulation from the smaller red/white wire, twist it together and fold the end over.
4. Locate the 2 wires running to each speaker-module on either side of the parcel shelf. O/S wires are blue, N/S are yellow, both have a grey stripe (-) and a purple stripe (+). Slide a small flat-screwdriver into the gap on the wire-connector and remove the plug.
5. Cut the speaker-wires about 3" from the plug and strip the four ends.
6. Twist the ends of the wires from the plug and one of the RCA-cables together for each side - the grey-stripe wire going to the small red/white insulated wire, the purple-stripe wire to the large bare wire.
7. Join the wire from the plug/RCA back to the speaker-wires using crimp-connectors or solder, wrap a little elec.-tape around the RCA wires to keep the two separate and seal the join with heat-shrink tube.
8. Run the RCA wires into the boot through the gaps in the near edge of the parcel-shelf and tape them into place along the crossmember at the back of the boot.
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GETTING A REMOTE TO THE AMP:
The amp wont power up without receiving a remote signal (RM) and the BMW Business stereo doesn't have one of these either, but luckily the RM signal is just a 12v line from the head-unit that tells the amp to turn on and nothing complicated, so you could just wire a cable from the input holding the red Power cable on the amplifier, or directly from the positive terminal of the battery itself, straight into the Remote terminal on the amp. This would make the amp power up all of the time though, so unless you wire in a switch that relies on you remembering to turn the amp off every time you stop, then we need to tap into the ignition switch somehow.
The CD-changer may be an obvious choice, but it turns out that this is live all the time so the CD carriage can still be ejected when the ignition is off. However, the Rear Parking Sensor has a remote 'on' signal from the ignition switch, known as 'Unloader Relay K3', so this is by far the best place to get our signal from and its located just next to the battery.
1. Pull down the right-hand panel in the boot to expose the battery and aux. fuse box.
2. Remove the 3 bolts along the bottom of the aux. fuse box bracket using a 10mm wrench.
3. Snip the cable-tie holding the wiring-loom to the base of the aux. fuse box.
4. Push the studs clear and tilt the aux. fuse box forward to expose the wires running into the back and locate the Purple/White cable that runs into the Unloader Relay [the 3rd relay from the front].
5. Snip the Purple/White cable and splice in the remote wire, or use a two-way crimp connector.
6. Run the remote wire through the aperture in the boot-lining and out of the top by the suspension-turret, so the wire doesn't exit around the panel door/hinge.
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ADDING AN ON/OFF SWITCH TO THE REMOTE:
The remote turns off with the ignition, but it might be nice to have a way of turning the subs/amp off while driving, without having to fade the rear-speakers out too.
1. With the rear-seat out pull the rubber-bung out of the gap where the wires pass through the bulkhead.
2. Remove the centre-clip and the four plastic screws from the rear boot-lining and pull it clear.
3. Gently bend the corner of the boot-lining down until the gap behind the rubber-bung can be seen.
4. Run the remote wire from the unloader relay through the bulkhead into the cabin.
5. Gently prize the storage-trays from the back of the centre-console and inside the armrest. Slide the rear air-vent down and remove the 2 screws holding the armrest and 2 holding the rear centre-console bracket in place, using a crosshead screwdriver.
6. Lift the carpet on the transmission-tunnel and the rear centre-console bracket enough to run a wire under and up the side of the rear storage-tray/air-vent to the bottom of the armrest.
7. Drill a hole in the armrest storage-tray to mount a toggle-switch and connect the wire to one terminal. Be careful the switch doesn't foul the air-vent.
8. Run a wire from the other switch terminal back through the carpet/bulkhead to the Remote input on the amp.
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ADDING AN ON/OFF SWITCH TO THE REMOTE:
The remote turns off with the ignition, but it might be nice to have a way of turning the subs/amp off while driving, without having to fade the rear-speakers out too.
1. With the rear-seat out pull the rubber-bung out of the gap where the wires pass through the bulkhead.
2. Remove the centre-clip and the four plastic screws from the rear boot-lining and pull it clear.
3. Gently bend the corner of the boot-lining down until the gap behind the rubber-bung can be seen.
4. Run the remote wire from the unloader relay through the bulkhead into the cabin.
5. Gently prize the storage-trays from the back of the centre-console and inside the armrest. Slide the rear air-vent down and remove the 2 screws holding the armrest and 2 holding the rear centre-console bracket in place, using a crosshead screwdriver.
6. Lift the carpet on the transmission-tunnel and the rear centre-console bracket enough to run a wire under and up the side of the rear storage-tray/air-vent to the bottom of the armrest.
7. Drill a hole in the armrest storage-tray to mount a toggle-switch and connect the wire to one terminal. Be careful the switch doesn't foul the air-vent.
8. Run a wire from the other switch terminal back through the carpet/bulkhead to the Remote input on the amp.
