if it were me, my first move would be to check -all- the power cables for ANY (less than 100K ohms) short to frame, and eyeball all the cables to be sure none are soaked in corrosion, replacing any that are soaked, or have ANY damage to the insulation. next I would test to see if I could tell if the power was bleeding through the directional switch because of moisture or corrosion inside the switches (does removing the power [common] wire from the switch allow the current flow to stop as it should, does removing the normally open wire from the switch end current flow as it should? do the same on the reverse switch as it may have a circuit that runs through the rev switch normally closed side that should open up when the reverse switch closes, for opening the power to the forward circuit.