I am going to assume you are talking about the plug that comes from the speaker box, I looked through my files and I don't have a clear photo of the wiring for the speaker box side.
I think you can safely assume the two black wires are grounds, you can test the other two wires by using a 9V and listening for whether the left or right speaker makes a sound.
Once you identify the Left +, Left -, Right +, Right -, take a look
here for a picture of the headunit wiring then use the Becker pinout diagram (in the manual) to determine which color is left and right. You will have to guess on the grounds, so I would probably test the wires before inserting them into the plug.
Good luck!
