Help please, 1982 300D turbo. Getting 12 volts at fuse 4 accessory side of radio but not at the other fuse . Question is where to start. Good power from Battery . Acc side powers my XM unit but no power to head unit. Does anyone have a good wiring diagram or a suggestion . Could it be ignition switch? Not getting power at yellow battery wire to stereo. Trying not to pull everything apart if I can. This thing has had multiple mechanics work on it in the past, so I have been fixing a lot of shortcuts done by idiots. PS new alternator a couple weeks ago. Also new crank sensor to get my tach working. Any help appreciated