Here is a typical schematic of the 2004-2005 LLY 6.6L GM Trucks Duramax Fast Idle Wiring Diagram. All of the wiring required to operate fast idle is present. 2500 and 3500 series trucks need to have the 488 circuit added from C106 (located on the side of the underhood electrical center) to the ECM as shown in the schematic.