Well, I appreciate the help, guys.
I test it by leaving the doors open with the key off (so I can hear the radio on) and then going through all the fuses, jiggling wires, etc. looking for the radio to go off. Of course, if I remove fuse #13 it goes off. If I remove #12, it goes off too. That is the one that blew and caused my door lock problem. My replacement of that one is when this whole thing started.
Strangely enough, when testing this (and touching nothing) sometimes the radio will cut off and on (like you are wiggling a wire with a short).
To sum up -- an open diode won't let this happen (door open key off, seats DO work). Are you saying then, that this may NOT be it?
1994 C280, dark green metallic
1991 190E 2.6, Black
(sold to another forum member)
2003 Chevy Tahoe LT, Redfire Metallic
1989 Mustang GT Cobra Convertible