1999 Ford E-Series Van 3 speed fan switch

Check for voltage at all of the resistor terminals with the switch set to any speed other than the fastest one. If you find 12 volts on just one wire, there's either a bad connection on that terminal or the internal thermal fuse on the resistor assembly is burned open, (or you have the wrong resistor assembly). Try the old resistor assembly again. If the fan runs on all speeds, there's no need to replace that unit.

Repeat switch failures are usually caused by a tight fan motor, but that will be aggravated by using the fan on the medium speed a lot. The highest speed naturally draws the highest current, which results in heat buildup on switch contacts and connector terminals, but yours uses a relay for the highest fan speed. That reduces the stress on the fan switch but not on the ignition switch.