Update from the W126 CD manual (should be relevant since the stereo units are the same in the W126 and W124 models of this generation) -

connector 1 (picture 2):

- brown = ground
- black with yellow stripe = hot when key switch is on
- gray with violet stripe = hot when lights are on
- red with white stripe = always hot

connector 2 (picture 5):

- blue = antenna

connector 3 (picture 3):

- black = left (-)
- black with red stripe = left (+)

connector 4 (picture 3):

- black = right (-)
- black with green stripe = right (+)

That should be enough. Get to work!

Sixto
95 S420
87 300SDL