Aux battery relay
 

I have a 91 C2500 Silverado X-cab with the auxiliary battery. The auxiliary battery is connected to the truck's charging system by a heavy duty relay. the coil wires that fire the relay go to the fuse panel, but are not marked on the fuse panel. I'm trying to figure out what it is, that closes this relay. All this is factory, so it's not Bubba's contraption. If this relay is not closed, the auxiliary battery never gets charged. I have all the electrical schematics, that show all the wiring for this auxiliary battery circuit, but it doesn't show what fires the coil to close the relay.

Any help is greatly appreciated.

Thanks,
Steve