Why does my 91 ford econoline over heat when I turn my heat on?

I've been working a similar issue with a friends truck. When the heater blower motor is off the engine temperature gauge reads normal in the middle. But when you turn the blower motor on the gauge goes way up to the top. The engine isn't really overheating. There is an electrical problem that when you turn the blower on it is affecting the temperature gauge and making it go all the way up off-scale. So far we've found that the blower motor speed control resistor is bad. We replaced that and the speed control works now, except for the low speed setting. I suspect there is a bad ground somewhere and the bottom end of the resistor element isn't grounded anymore. As such too much current is going through the ground in the dash, which is affecting the gauge.