First of all, the round connector you found will have one of three functions: equaliszer, CD changer or amplifier. Either way, I would forget about it. You will have two multiplugs, one of which has all the power stuff and front speakers and one for the rear speakers. If you do not, someone has played about with it all. First thing is to trace that round cable to the amplifier and disconnect that. If you have rear speakers and amplifier, chances are that the wires for them are at the amp. Using a multimetre, figure out what wire has permanent power and which has ignition on power. Ground will be black. Then, find out which is the antenna up cable. There will be one to control the dimmer part of the radio. FORGET THAT CABLE, can cause serious damage. The speaker wires will be pairs, i.e. front left might be white and white/black or white/grey and so on. You can figure that out buy simply connecting them to the stereo. You need to eliminate the power cables first because a) if you apply 12 volt to speaker out, you will cause damage and b) you can't play the radio to find the speakers if you have no power ;-)

Hope that helps.

George
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George Holmer, Belgium, Europe; 1987 745 GLE Turbodiesel Intercooler (D24TIC/M46 190k and 1988 745 Turbo Intercooler (B230FT/M46) 290k; 1988 745 GL (B200E/M47) 190k