Factory 12V radiator fan?

[screwballl]

My 91 has an electric fan on the front side of the radiator and even on the hottest and most humid days here I have never seen/heard that fan come on. Even when sitting at the bank drive thru and the temp gauge is sitting around 190-200 instead of the usual 150-160. There is a wiring harness but because it goes into a split loom with 4-6 other wires it is impossible to trace it without tearing apart the grill and everything else along the way. Is it hooked to a switch that determines when the engine is under more stress than usual and then turns it on?? I know it does work because I unhooked the harness and hooked the battery directly to it and it works fine.
Does anyone have a wiring diagram for a 91 R2500 burb (with 454 since other 91s with 350 dont seem to have them)?
If it is hooked to a switch that isn't working, either I can replace the bad OEM switch or I wonder if I can use an existing unused toggle switch inside the cab to turn it on when needed without causing error codes.
Also if possible does someone have a vacuum diagram as well, I am looking to replace the vacuum lines to be on the safe side but just in case I mess them up.