It's a hot lead that is switched on via a relay inside the ECU after the ECU gets the signal from the water temperature switch on top of the thermostat water box and maybe also a signal from the A/C refrigerant pressure. It's been a while since I looked at the diagram to be certain of the inputs, but I think I'm remembering it correctly.

There is a short lead that will be attached to the compressor which will reach the one coming from the fender area.

Frank

'89 [color:"white"]G-Raider[color:"white"] [color:"black"]Supercharged 3.0L, MegaSquirt 2, lockup A/T, 2.5" exhaust, 172k, Cibie H4s/Oscar SCs, Hella Micro DE fogs, Cobra CB, Superwinch hubs, LSD rear/Aussie Locker front, Bilsteins, Lifeline AGM, Rust-Oleum