i did this to (kinda but not auto), but i wanted to controll when my fan was on and off, i just cut the factory plug for the cooling fan and butt connected my own wires and wire loomed it to a relay and ran both negatives inside the car to a toggle switch, i had it on the positive side but it was blowing 30A fuses (i was getting frustrated because i read on here that all u need is 30a relays and u should be fine but it draws to much current to put a switch after the relay, wiring up the relay on the positive side) So i called my friend up, he came over after overlooking everything i did (another electronics tech) he sugested to use the ground to cut off the power and it worked and i havent blown a fuse in almost 1 yr ?? very cheap to do, i bought a toggle switch (4-5$),30A universal relay in the fog light section of auto parts stores (15$) and i ran 10 AWG high stranded audio wire for a nice flow(15-20$ ) (best thing to do is get a 8AWG amp kit from walmart
), i might redo mine with 8 AWG though and bought some crimp connectors (5-7$).. wasnt to bad at all, i noticed a BIG BIG difference in cooling temp with just my low speed fan on 90% of the time (thats all i hooked up to the relay) and all u should need because thats all i have been running and had "0" problems with temp, havent even used my high speed fan because i didnt hook it up, but if u do alot of people say u need to run the low speed fan first then disengauge the low speed and turn on the high speed fan on so u wont burn out the fan..
my temp with a flushed/checked radiator with, replaced water pump (3-4 yrs old ??) new 180 degree thermostat still idled at about 220ish, and i hooked up my switch now i barely go over 200
sometimes 5 degrees cooler on colder days or on really hot hot days getting on it about 205 tops
i love it.. i feel much better now about my cooling system,
**idea, now i have seen some temp switches for 180 degrees i think at mn12performance ? now if u hook that up inline it should kick on at 180 automatically if u wire it right, i might look into it and do that when i upgrade my wire to 8 awg, but i want a fullsize aluminum racing radiator for a mustang (make some custom brackets to make it fit) one on ebay for like 200, they say there equivalent to a 4 core brass
** there is a negative, i have forgot a couple times and forgot to put on my switch and cought myself after, luckly nothing happened my engine, when i looked my gauge read 250
Alex L