If you ar erefering to the one that has the little bullet plug that is your (its working light) in your coach, it hooks to your brake switch on your brake pedal. I don't know on your car what wire it is, but with a tester it would be easy to find. The brake switch is on the brake arm under the dash. Find that, and test wires with the brake pedal depressed, the one that is hot when applying the brakes it the one it hooks to.
Hope that helps.
Command Master Chief (USCG, RET)
Full Timing in a 2007 American Coach Eagle 45H towing a 2015 Ford F150 4x4 hauling a 2015 Anniversary Edition Honda Goldwing