While the problem could be a bad connection at the fan switch or a bad fan switch, it is unlikely. I suspect there is a burnt thermal limiter fuse in the heater resistor block. The thermal limiter fuse protects the resistors from overheating due to high electrical current flow. Unplug the connector, remove the two screws holding the resistor block, and pull it out of the housing. The block is not much larger than a deck of cards.
Installation is done in reverse order.
Sometimes these fuses just fail, but if a second one burns out, then the heater motor is likely drawing too much electrical current and also requires replacement. You can test the heater fan by operating it directly with a jumper wire with an 20 amp inline fuse in it. If the fuse blows, then replace the heater motor. More articles by Jim Kerr Jim Kerr is a master automotive mechanic and teaches automotive technology.