GROUNDING OF THE AC POWER SOURCE MAY BE DEFECTIVE (OR THERE MAY BE NO ELECTRICAL GROUNDING AT THE AC POWER SOURCE).
ANYONE COMING INTO PHYSICAL CONTACT WITH THE TRUCK WHILEIT IS CONNECTED TO A NON GROUND AC POWER SOURCE MAY RECEIVE AN ELECTRICAL SHOCKSUFFICIENT TO CAUSE PERSONAL INJURY OR DEATH.
REWIRE ENGINE BLOCK HEATER TO ASSURE PROPER GROUNDING.