PS if you get the cheap one at the part store spray the coils with some clear aerosol sealant.... That will keep the rust and water off the coils for a few extra years. You don't need to do it heavy... just a light dusting to keep moisture off them.
The only reason I spray mine with a light layer of clear coat (or wild dog 40 depending on what i have at hand). It helps keep the salty wet conditions off the coils that go inside the heater housing. I have had a couple fail within 3-4 years on me, so I coat the coils to prevent the coils from rusting so quick (it could be b/c I am buying the cheapest resistors too ).
That being said the clear coating only helps to keep the resistor coils from rusting... You shouldn't be having a resistor fail within a month! How do you know it's the resistor failing???
How do I know its failing... When the fan only works on high, I replace that part and it works at all sopeeds. Lol. The truck hasnt been used much since the part was replaced. But I did notice some noise from the fan motor the last time I drove it. i will have to take a look.