I've got a real gremlin hiding in my '89' 240 wagon-the rear defroster doesn't shut off like it should after it's been on for a while.I've swapped the relay and the switch,cleaned the fuseblock&replaced the fuses,checked the wiring at the hinges(their both getting bad,but everything works);I've pulled the fuse and the damn thing still won't shut off(the O/D even still clicks in& out).Last night,I even tried unplugging the harnesses with no effect.There seems to be a feedback problem, but I can't find it.I also noticed that the A/C pump doesn't seem to be cycling when the A/C switch is turned on.In nearly 30 years of working on Volvos, I've never come across this problem before-

Any ideas or tips to help me figure this out?

When mine would not shut off and a few other gremlins showed up only when very humid and cool, a sojorn under the trim panel over the fuse panel was required.

Disconnect the battery. Remove the panel that covers the fuses and all the associated wiring. It is a pain to remove but will come out in a similar fashion like what covers the RCU on the right side.

With some elelctrical contact cleaner and a hard bristle tooth brush, remove one termianl at a time, clean everyhting, tighten with pliers and reconnect.

Then find the ground screw undewr the left tunnel carpet and do the same.

Sounds like a hot wire is leaking into the defroster circuit. Check all aplicable wiring through the harness and see if you can locate a section where a bunch of wires have frayed and are contacting each other. Baring that wiring problem, it would probably be more involved than that - I would simply disable the defroster from the dash switch (unplug it) to prevent battery drain and overheating.
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-Flup, 240DL Wagon, 2 subs in the trunk, Dancing Bears Sticker to top it off :)

I had something similiar with my 1990 Wagon even though the tailgate wiring had been fixed. Adding an additional GND from the tailgate to the body fixed the problem.

My guess would be the wiring in the tailgate hinge. The supply line to the defrost runs through the left side hinge. I would pull that wiring and splice in some new. Good luck... electrical gremlins can be a real PITA.

-J