First - make sure that the blower motor is properly grounded through a short 12-gauge wire that securely connects bare metal on the blower motor mounting flange to bare metal on the cab. Also, make sure that the cab is properly grounded through the braided 5/16" strap that connects the firewall to the back of the passenger side cylinder head.
Second - check for power to the blower motor with a test light or voltmeter. If there's zero or very low voltage at the blower, inspect the selector and blower switches behind the dash on the control head assembly (and their respective harness connectors) for any signs of heat damage/melting/burning. The Pack-Con style terminals don't manage high current very well and have a tendency to concentrate heat and distort/burn. If there's any evidence of heat damage, replace the switch and connector and coat the new terminals with dielectric grease or antioxidant.
Please, let us know the outcome and whether you need further assistance.