BMW Repair: BMW Stereo replacement, wireing diagram, single cd player

Question
I have a 1986 325 (2-door). I am replacing the original stereo because it has a short with a Sony single cd player. There is a connection from the old stereo labeled battery with a white and a yellow wire that plugs into a connector that has a red/gray wire and a red/green wire. Everthing is working by me not attaching the red/gray wire but the stereo plays at a much lower volume. If I turn it all the way up it is still very low. What are the combo colored wires to? Is there a wire spec sheet I can look up? What am I doing wrong with those wires?

Answer
I don't know if this car came with an amp but if it did it would be in the trunk above one of the wheels behind the power antenna i'm not sure this is what it goes to.  
to be more sure get a wireing diagram from haynes or check some of the tech websites:
http://www.roadfly.org/bmw/forums/
http://www.bmwe30.net/
http://www.bmwworld.com/repairs/e30.htm
-cheers