964 AC and power window electrical problems

[Revrider]

I have a 1991 C2 coupe and have recently been experiencing unexplained failures of the AC (or heater) fan to work. This is sometimes in tandem with the power windows and sunroof not working. It seems quite random. Sometimes both fail to work at the same time, which is hell on a hot day with no AC and no way to roll the windows down.

Fuses all check out fine, and a day in the garage usually "solves" the problem.

Just replaced both window relays and the fan relay, hoping that would fix the problem, but it didn't.

Suggestions for trouble shooting this would be appreciated. I'm no mechanic, but I'm good with my hands and pretty good at structured problem solving.