After 30 years I tracked down the car I had in high school and am now restoring it. The latest issue is the heater blower motor. Someone had replaced the original 3 wire motor with a 2 wire. It did work, but of course only had one speed. I pulled the heater box, replaced the core, put in the correct 3 wire motor from C J Pony, resealed the case, and reinstalled. When I tried it, it blew the fuse. Thinking maybe I had a clearance issue with the squirrel cage, I pulled it back out. It's fine, no clearance issues. I had tested the motor on the bench with a battery before I put it in the car, and it worked fine on both low and high speeds. After mounting it back in the car, but before I connected the wiring, I put in a new fuse and checked for 12 volts at the firewall red and orange connectors. With the blower switch on low I got 12 volts on one wire, and with the switch on high I got 12 volts on the other wire. So I reconnected the wires and tried it again. Blew the fuse. Next I went back to basics. With the heater box still in the car, I eliminated everything and used jumper wires to go directly from the motor to the battery. Black wire to negative, red or orange to positive. Nothing. Then I got a second battery, set it on the fender, hooked it up the same way. Motor runs fine on both low and high speeds. After scratching my head, I went back to the car battery, and yes the car does start and run fine with that battery. I disconnected the positive battery lead and tried the jumpers again. Nothing. I put the positive lead back on and disconnected the negative lead. Motor runs fine on low and high speeds. What's going on?