I recently bought a 1994 850 with the SC810 HU. The front dash speakers and the rear deck speakers do not work. I thought it might have been bad speakers but I replaced the front dash speakers with new ones and they still don't work. I suspect that the previous owner may have disabled the amplifier <it is equiped with one>. However, I can't find where it is located. Ive looked under the drivers passenger side kneepads. I've head it may be located under the radio behind the cigarette lighter. Does anyone have photos of the location of the amplifier and the amplifier itself. I have feeling I wouldn't know what it was even if it were staring me in the face.

Actually, I doubt you have an amp. It is possible, but not likely. Pull your HU and see if there is anything plugged into the 6-pin DIN socket marked "AMP." This is the easiest way to tell.
A simple way to find it, is to follow that cable. If it is there, it is located in front of the heater core, on the tunnel right behind the radio and A/C controls.

To remove: Pull the carpet back, remove the two black plastic covers, you will then see the amp mounting bracket.

Also, get a DMM and check to see if the speakers are even getting any power. It may not be the amp (if you have one) causing the problem.

Thanks. I pulled the front speakers out and discovered that the magnets on both fell off. But the rears seem to be intact. I will check for power going to these and let you know the results.