Originally Posted by P2164
.... Another problem on early cars is all the current for the blowers also goes through the key switch.
There are a couple factory-fixe documents related to the blowers; it's a confusing comedy of errors with one fix undoing the previous one, etc.,. Attached are the related docs. The primary fix does part of what Jack did (i.e. without the cool PWM motor controller) to protect the switch terminals. It involves wiring in a factory-supplied pair of relays so the poor switch isn't trying to power the motors.
So to get them in historical order, first read this: 0601GT Engine Run On.pdf
Then this (which is where the switch overload fix occurs): 0702aGTB HVACBlower Fan Relay Retrofit Supplement.pdf
Then this: GT40 Fuse 12 Service Bulletin.pdf
Hopefully you come out of that with reliable blower motors that don'e keep you from turning off your engine.
I just remembered: here is my story implementing the primary fix: Notes re SPF Service Bulletin 0702GTB HVAC Blower Fan Relay Retrofit