Wiring>>A/C to fan, low, high??... not stock......

Here's a quick run down of my setup, just for clarity......I've got 3 fans, two small ones on the front of my radiator pushing through, and one big one on the motor side, pulling... the small ones are wired to the battery and via a thermostat probe in the radiator, they can come on once the engine shuts down, they're set high and come on when it gets really warm, they don't draw a lot and I've heard them stay on for several minutes after shut down, no problem. The other one is wired through an ignition power source, and essentially it comes on as soon as the thermostat opens up and get the radiator warmed up..and won't come on if the car is off. No problems there, monitoring my coolant temps throughout all circumstances, things are great.... however..... trying to get my ignition sourced fan to turn on when I turn the air on has gotten me a little baffled. First off, does the high, or low fan come on in a factory situation when the A/C is on? What happens to turn that fan on when the a/c is turned on? Is a relay tripped and it grounds the connection to the fan? Or is it sending 12 volts? I've been in the garage turning the a/c on and off, probing the factory fan connections trying to see if any of them throws 12v, i get nothing. I could easily run a toggle switch into the cabin and turn it on when I run the a/c, but I'd rather not. I just want the fan on because the pressure gets really high before she's warmed up enough to kick on the fans. Anyone have the magic wire I need to tap into for either ground or power?....