I just purchased the Upfitter Switches and wiring harness, and installed it last night. There are a Red, White, Blue, and Black wire in the engine compartment, that feed through the firewall to a bundle just under the steering column. These wires were meant to be used with the Upfitter Switches, to control any additional accessories. The bundle of wires located near the steering column is called the Customer Access bundle, or CASC. In this bundle are a bunch of other wires that are "capped" off. Amongst them, is one thats Purple/Light Green, which is the High Idle Circuit that you connect to the AUX4 switch. The others Im not sure. Knocked down several birds with one stone on this one. Got the Upfitter Switches installed, installed my rear off-road lights (For connecting/disconnecting my travel trailer in the dark), and did the High Idle Mod, all in about 2 hours.