1983 c20 blower only works on high

[ZXT]

Alright, here's the deal. I have an 83 c20 (4 door, long bed, 454) and the blower only blows on high. Now, you're thinking it's the resistor - it's not. Replaced it twice, and it have tested the continuity. The fan switch and relay have been replaced as well. All that I can think of now is that there could be a short in a wire somewhere and that's causing no voltage to be received to the blower.

Any thoughts? What wires should I trace out? Any schematics?

Thanks!