Don't have a diagram, but installed a newish HU in my '85 w123 a few weeks ago and copied all of the wiring from it.

Power set:
ACC - Black
B+ - Red
Illumination - White
Ground - Brown
Power Antenna - Blue (covered in plastic)

Speakers: (Similar colors are assigned to each speaker, IE Brown + Brown/Stripe)
Front L - White
Front R - Brown
Rear L - Black/Yellow
Rear R - Black/White
This is all given you are getting rid of the stock POS fader.

I would check all of these with a multimeter before hooking all of them up, just in case. Hope it helps.

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My Primary Driver - '85 300CD - 4-speed conversion, 2.47 rear, lowered, euro headlights, rebuilding (not restoring so much)

Wife's - '08 Saab Sportcombi Aero

Riding a '03 Yamaha Warrior