Ok, here is an update with additional symptoms.
Now, when the door is open and key is off -- sometimes the radio plays and sometimes it doesn't. Sometimes it goes on or off by itself.
Also, power seats DO still work with the doors open and key off. However, sometimes with the door closed and key running the power seats won't work. If I open the door and touch a seat button, then close the door it will then continue to work.
Whether diode or relay -- don't know that yet -- it seems like it has partially failed / short / bad connection.
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