Mike-
the factory switch was designed to handle the "designed" load of stock wiring demands. towtrucks as an example, gets quite a few more lights than your stock pickup and the result is every light bulb you add, the amp demand goes up. Unlike GM who put a relay on just about everything, ford still doesn't to this day. The whole amp load on the lights goes right into the switch. if there is any problems, the switch will either melt or worse yet, go up in flames. That is the #1 cause of death to our diesels. Relays, take the load off the switch and can handle the amp load better. Depending on how much you want to spend, the sky is the limit for the amp load. My favorite is the circuit breaker/ relay assy that AWdirect sells. Very pricey but keeps the truck from burning to the ground.