<
I'm quite sure this will help the motor stay a bit cooler in hot summer traffic weather, plus it should not be too hard to do, I have all the components already, >>

If you simply want to control the fan manually from the cabin, you should just use the existing circuit. [ the relay, wiring, fusing and complete circuitry are already there. Just put a switch in the cabin and run the 2 leads up to the a/c drier high pressure switch.
When the switch is On , the a/c aux fan will come on using the stock components...cost you $2-3 ...