However hard it is to get at, it needs inspecting and better to remove and clean it up. Gauge panel comes out, dog house off or help from the engine compartment. It is a convenient (at manufacture ??) tie point, bringing negative returns back to the batteries negative.There is no body ground on the hmmwv. The spot on the firewall is a connection for the stuff in dash. It just happens to go thru the body.
If that connection is hard...wait for the other problems, CAMO