E36 Alpine stereo system

I removed the Alpine head unit and 6 disc CD changer two years ago from my
'96 E36 328 iS (for a new, aftermarket system). Recently I tried to
reinstall it and it won't work. The display reads "ERROR". I'm not sure if
the problem is at the head unit, the changer or both. Is there some sort of
code I need to use before the system will work? Both units worked perfectly
when they were removed. TIA for all of your suggestions.

I can't be sure your '96 radio is the same as in my '98 E36, but when power has
been restored the display says "CODE" followed by 5 spaces for you to enter the
code. It is part of the anti-theft system. I recently replaced my battery and
went through this. The code is provided when the car is new. If you don't have
it, I believe a dealer can get it for you if you have the radio serial number -
and presumably some ID and proof of ownership of the car.