My 300d, and my sdl both have a front/rear speaker selector next to the window switches. I am assuming yours has one too, and it's probably your problem. I have had to do away with that switch in both of my cars.

If you take it out, the left bank is for the left speakers...and vice-versa. I just verified all the wires go to the right speakers, then soldered them together, effectively doing away with the switch.

You won't be able to fade, but all your speakers should work.

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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