Heating wiring question

[Marshall50]

Hey guys, brand new to the forum. I've read many threads but can't seem to find what I'm looking for. Hoping someone can help me out.

I have a 79 K10 with a new 3 speed heater only fan selector. I recently swapped out the heater only box with an ac/heater box that someone donated. I don't care about the ac right now but only want the heater and defroster to work right now. Here's the problem I have... the fan selector is 3 wire and the resistor is 4 wire. First of all is this even possible to use the non a/c selector switch? I don't want to have to buy another new switch just yet unless I know for sure I will want ac in the future. Currently, I only have the blower motor working when the key is on and the selector switch is on hi. There is nothing on the 1st two speeds. Any help is appreciated.