well they're right about the internal battery...
and they're right about the error codes (see below)

"E2"or "E3" (Without CD Changer)

1 Input Power to the RECEIVER UNIT (Trunk mounted) is missing, Check with volt meter to test for 12 volts
2 Fuse of the RECEIVER UNIT (Trunk mounted) is open, Replace it with 7.5 amp fuse.
3 The BUS CABLE (gray colored) that connects to HEAD UNIT(dash mounted) to the RECEIVER UNIT(trunk mounted) is not connected or is defective.
4 RECEIVER UNIT(trunk mounted) is defective.

"E2 or "E3" (With CD Changer)

1 Input power to RECEIVER UNIT (Trunk mounted) is missing, Use a voltmeter to test for 12 volts
2 Check the RECEIVER UNIT for an open fuse and replace with 7.5 amp
3 Input power to the CD CHANGER (Trunk mounted) is missing, check with volt meter to test 12 volts
4 BUS CABLE (gray colored) is defective or disconnected,
5 CD CHANGER is defective, Disconnect the CD CHANGER from the HEAD UNIT, if the E2 code disappears, Replace the CD CHANGER,
6 RECEIVER UNIT (Trunk mounted) is defective.

I hope this helps you trouble shoot this a little
now then, if you don't want to bother with the factory setup, with a $200 budget you can get a new aftermarket in dash cd player installed, and forget about it....

ps, you on LI too?

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95 M3 (US)
94 318is (EURO)
87 190E 2.6