Heater fan blows fuse ...

The other day my son and I replaced the heater core in my '74 Duster. We seized the opportunity to check out the blower motor. When I flip on the fan switch it blows the fuse. I always assumed there was a mouse or some crap cloggin' up the fan ... there was a little mouse nest in there, but not really jammin' up the fan at all. We hit the fan with some juice and it seemed to work fine. Cleaned all the connections/grounds/etc., put it all back together and tried the fan switch. No fan and it blew the fuse. Any ideas out there what could be causing this ? Any help greatly appreciated. Thanks !!! Tony