Fan

My resistor keeps burning out on the fan that provides the cab cooling and heating. It only runs on high. We have a 99 dodge intreped. An auto electric shop was unable to help me. I have installed two resistors new from the dealer they work for about a week. The repair shop stated the speed of the fan is controled by a ground. The fan works great but only on high. I have checked for condensation from the cooling unit that it might hav been going on the resistor but it is dry. Any suggestions.

thanks.