Thanks for all previous comments.
BTW the car is a sedan (no rear hitch problem).

I removed the OD relay, put in a new fuse, and started up the car. The rear defroster works, no fuse blowing. Hence problem is in OD circuit, in the relay or in the wiring to the switch & solenoid.

I popped open the relay, no signs of shorts, no cracked-off solder, no solder overheated or crossed over. There were some signs of cold solder joints so I resoldered the complete relay while at it. I am though 99% confident the actul short problem does not lie in the relay. Next step is to look under the car, wiring from shifter (auto) to solenoid. Time to get messy.

Intersting thing to add: When this happened this morning, the fuse blew. I stopped the car, put it in park, replacxed the fuse, and right away when the key was turned it blew again. This would indicate that it is likely not an "intermittent" short, given that I stopped the car and put the shifter to park. Question: Does the OD silenoid ever short internally?

Thanks in advance,

Greg Mustang
Montreal - Ottawa
Canada