Each coach is a little different. If you have schematics the harness will be called out. My 99 Windsor had about 8 spare wires, one end under the side console near the allison keypad and the other end ps engine compartment along upper frame rail. If you can find one end use a circuit tracer to find the other end.
2005 Safari Panther Diamond IV Cat C13
2013 Avalanche Toad