I just installed an external amp to run a sub box in my 90 535iM by tapping in to the factory wiring at the factory amp:
Pin # 12: Left pre-out rear + (Yell/Blk)
Pin # 10: Left pre-out rear - (Brn/Orange)
Pin # 5: Right pre-out rear + (Blu/Blk)
Pin # 4: Right pre-out rear - (brn/Orange)
Pin # 9: Remote power (only supplies power in ignition position 1 or 2)
Fairly simple to do.
1. Remove felt cover from left side trunk (you do not need to remove the front felt panel attached to the back of the rear seat, the left felt cover will swing inwards enough to get at the amp).
2. Remove 4 phillips screws (2 top, 2 bottom) attaching the factory amp to the inside of the rear left bodywork. Pull amp out, pinch tabs on white plug while using a flat screwdriver to gently lever the plug out of the connection
3. I used wire taps to get the signal from the head units L/R pre-outs (at the white amp connector) before they enter the factory amp (there may be a tidier connection method of tapping?) by crimping them at the same time on the correct factory wire and new outgoing external amp
wire (you may need to strip some of the factory harness tape to allow enough play in the factory wiring to attach the somewhat bulky wire taps - stagger the taps so the don't all end up at the same position relative to the white connector). I used a short piece (about 8 inches) of L/R RCA wire for the pre-outs to my new external amp, stripped on one end for tapping, plugs on the other for my new amp. Once the tap was completed, I used male to male adaptors to connect the tapped pre-outs to another RCA cable long enough to reach my new amp. If/when I want to remove the amp I will simply remove the male/male adaptors with the RCA cable, and tuck the hard wired pre-outs back with the factory wiring.
4. 12V amp power is easily accessible from the battery in a 535 (under the rear seat). I mounted my amp underneath the rear deck, plugged the new L/R RCA's in, attached the remote power, ground, and speaker outputs.
I used a Rockford/Fosgate Punch 60 with a Blues isobaric sub, (both I had kicking around from my "car audio" days)it sounds great. In the future I will likely build a smaller custom modular sub box to attach to the ski bag framing, something that can be easily removed if I want to put the bag back in. I will post this project if/when it happens.
I would suggest running some sort of signal through the ground and hot wire of the newly wired L/R RCA (to detect with a meter of some sort) just to be sure you have a good conection. Do this before you reassemble everything.
A short while ago I installed a Pioneer CDX M40 CD changer which I also happened to have on hand from years ago, screwed around a lot to get it to work. Long story, but don't use the factory pre-wired CD changer cable, the config is different. I used the cable supplied with the Pioneer changer and it was plug and play, other than supplying remote power and a ground.
Hope this helps someone.