Try going to http://members4.boardhost.com/beckerboard/ and post a question
there. I have found it to be a very helpful site.

Becker radio of the 80's are two channel radio. So all you got is left
positive/negative and the right pos/neg. You cannot hook up new radio to
these wires... the load is too high, will sound crappy if you do, and ruin
your new head unit sooner or later.

I am assuming you are reinstalling the original radio... so  you will need
to figure out which is positive or negative by measuring the voltage.

The other reason why you probably not hearing anything when you hook it up
is because the previous installer left your fader disconnected at the center
console... you know... by your window switches.

The reason someone cut the wires is because they put a today technology CD
head unit in place of the antiquated Becker tape/radio and took it out
before selling the car. Check the speakers and see if they are aftermarket
instead of the pathetic ones that came with the car. All aftermarket
speakers are 4 ohms, and o.e. speakers are bertween 2 and 3 ohms. Since
Compact disk has been available for 22 yeaqrs now, and nobody makes cassette
(hissss) anymore, most Mercedes owners have replaced the Grand Prix junk by
now. It can be removed from the dash in under five minutes correctly. The
best approach is to run new speaker wires to each location and bypass the
dirt-collecting rotary fader. All aftermarket cd head unites have electronic
faders built into them. P.