LeeA,
I'm not familiar with these rigs, but I think the resistor is for the fan motor. The other items in the picture on the site you linked are probably associated with the A/C. The poor old resistor just happened to be in the picture because that's where it lives, it's an innocent bystander.
Red58 is an expert on these trucks, maybe he will see this and shed some light on it.
Stuart
