Well, I think you pointed the finger on the culprit: the radio.

My neighbor had a Honda that drained over a few days and when we found the draining circuit it included the mechanism that turned the headlights out when they were on (like on a corvette). Oh, she said, that mechanism doesn't work anymore since a few days. The motor had shorted and now drained. So disconnecting the motor (she didn't want to spend the money to repair it) solved the drain.

If everything else on that fuse worked except the radio, my money would be on that for the culprit and I would start with that.
Good luck

__________________
Reinhard Kreutzer
1987 420 SEL (126)