assuming you've correctly tested and done all you have said, very likely something is grounded (shorted) in the circuit. probably a wire has worn through. trace each wire of the whole circuit and look carefully for worn wires, or anything else touching the frame.
also have you verified the plug wire isnt cracked, or the boot not making contact with the wire?
"In this world there's two kinds of people... Those with loaded guns, and those who dig." -- Blondie to Tuco