I had a simple write up I did several years ago, but unfortunately I wiped out the hard drive on my computer. Anyway, I tapped into the harness as was said above on the pass side fenderwell and routed to a DPDT 3 position switch. Basically all you are doing is grounding the relays. You don't need any resistors (at least on F-bod). The 3 posiition switch when wired correctly will give you low speed, high speed, and off (pcm has control). Unfortunately my car is at home and I'm at work, so I can't look at what I did (old age/memory loss, LOL). If you have a Helms manual handy you can look at the wiring diagrams and probly figure it out like I did.
Remember you are grounding the relays, NOT running a voltage to them, they already have a voltage supply, and the pcm turns on the fans by grounding the relays also.