It's tough to imagine the PBU drawing enough current to blow the fuse. That thing is just a printed circuit board inside - I think it'd melt before the fuse blows. BTW, how many amps is #7 - there are a very few things I haven't memorized about the 124...

I'd guess compressor clutch coil. Get a DVM and measure the resistance of that sucker. Use ohms law to calculate the current draw - should give you some idea whether the coil is likely to be blowing the fuse. It's easy to measure the clutch coil resistance by pulling the Klima relay. Just measure between pin #7 (or is it #5?) and ground - that's the lead to the compressor.

- JimY