I have a 1996 C3500 with a 454. IT has an auxilary battery from the factory that apparently has never been hooked up correctly. The solenoid that connects the charging lead from the battery does not have the wire that is needed to fire the solenoid which completes the circuit so the auxilary battery can take a charge. I have taken it to the dealer and they are stumped. Apparently the take out wire in a nearby wiring harness was not exposed so the factory never hooked it up. I can energize the solenoid by hooking a wire from a hot terminal and then I get charging voltage.
My question is does any body know where I should look for that wire, in what harness. I needs to have a female slip on connector to hook up to the solenoid. I have looked all over and can not find a loose wire that would be able to connect to the solenoid.

I need to be able to energize this solenoid after the engine starts or else if I energize it continuously the charging wire would not be heavy enough to help feed the starter during start up.
Any help would be appreciated.