Hello guys I have a 1993 GMC sierra k3500 and the other day I went to go out plowing and turned my heater on and it wouldn't work. So I took my volt meter and tested it and it was sending 12.60 with the truck off and about 13.40 with the truck on, this is at the blower motor itself. and I was putting my ground on the ground wire I put on before. So with that all good I thought it was probably the blow motor so I had a spare and I tried hooking it up to the same power and ground wire yet nothing happen... but yet if I hook either fan up to an external battery they both work? What could this be?