I checked the PBU first - it's only about 15 minutes work. I pulled the wood covering the PBU - two small, vertical screws just above the radio. A stubby phillips screwdriver is required.
Flip open the top of the left hand wiring plug. If memory serves, the blower control signal is pin 12 (third from the back in the left hand row - I can look this up to be sure if you need it). Monitor it with a DVM - should see voltage slowly increase from 0-5 volts when switching the climate system from off to EC. If you do, the PBU is sending a signal to run the fan.
Apparently there's also a plug in the engine compartment, somewhere near the brake booster, which gives access to the blower control signal. I was too lazy to look for it when I knew I could get easy access at the PBU.
The blower control unit is located directly below the fan, so even if it's the problem you have to pull the fan. I bet if you open it up, and give the fan a spin and/or rap with a hammer it'll start right up.
I understand George Murphy sells the correct replacement brushes for ~$20. I'm going to order a set of these for next time the fan stops. Guess I'm too lazy to work over a set of non-standard brushes to fit.