With all the experience here, can anyone help me with this?

[Streetglide2015]

I just bought a beautiful 64 C10 short-bed, which has been basically restored. It has Thrift Air heat, but the fuses keep blowing. Each time I replace the blown fuse, I switch on the fan (all speeds), it runs for about 20 seconds, and fuse blows again.
Since the switch, and the fan motor seem to operate, could this be a resistor problem? Is there a fan motor relay also?
I located the resistor, but cannot seem to determine the location of a relay.
Any ideas on what else may be the cause?

Thanks!