I go with the leaking defroster theory. Our 172M when new cooked the
top radio in the stack so much you could hardly touch the front panel
during the winter. I used the pressure outlet of a vacuum jury rigged
with some duct tape to force air thru the defrost system to find the
cabin-defroster leak. Problem solved.

Other stuff we found - The 172M also had stainless steel cooling
plenums on either side of the stack, that were connected via 1 inch
dia scat hose to a small air scoop outside the aircraft. Tape on the
plenums covered the areas that had no electronic gear. We removed the
tape to match the revised setup. The scoop had only a 3/16 dia hole
thru the fuselage skin for whatever reason. We also drilled that out
to a larger size to get more plenum cooling. In hindsight that
probably wasn't necessary. We have maintained the plenum system
religiously even though the electronic shops suggested it wasn't
necessary.

The electronic stacks don't take that much power nowadays that they
could dissipate that much heat.