I don't want to dogpile about using a megger but frankly I can't see what value it would be even if it could be used. Meggers are used to stress insulation to determine its condition. Using it in this case wouldn't provide any additional information over what a VOM would give. As to the problem, wow, even Hitler couldn't win with two fronts. Either get the wipers squared away or the leakage squared away. As to finding the leakage, make a copy of the wiring diagram from the FSM. Highlight the positive side of any feeders to individual components. Then start isolating them one by one. I have been down this road more than once and as shotgunning may be good for pheasants, it isn't for electrical troubleshooting.