It has to be a ground problem. The heater is grounded by the mounting bolts thru the firewall. Make sure you have a clean (no paint) connection to the firewall. Also make sure you have a good clean connection from the battery ground to the frame.
A quick test is to run a ground jumper wire from the mounting bolt to the batter ground and see if the fan runs, If so,you have found your peroblem.
Harold
