In addition to Steve´s help: On the vehicle, there´s a wire loom running down the bulkhead behind the commander´s seat. This wire loom disappears into the underworld to the left of the commander´s left leg (together with the refrigerant lines). Underneath the Humvee, this wire loom should have one wire going to the small electrical diesel pump, and one wire going forward along the starter, and then going up at the right front of the engine. Maybe follow this path, then hopefully, you´ll find the wire.
Mine was cut off at some point, however, but thanks to a fellow Steelsoldiers member, I was able to source the missing bit of wire loom.
