I realize this is a very, very old post, and I'm not 100% sure about this solving your problem, but . . . When you remove the motor mount, antenna wire and white motor control wire plug, and pull out the whole mast assembly to work on, you notice a short black grounding wire runs from the upper area of the mount (isolated in rubber bushings). This black wire attaches to one of two bottom screws that holds the motor onto the mount. This is how the mast gets grounded to the vehicle frame. When the mount is bolted into place, that screw is positioned at the lowest part of the mount (ie. closest to the pavement), in such a way that any water that runs down the outside of the mast or mount (or overflows out of the drain tube), will pour over that screw, and the wire hoop, thus corroding the galvanized wire mount area, the wire mount hoop, and/or the copper wire inside that black plastic wire. When I saw that nasty corrosion, I used a metal brush to clean up the wire hoop (ie. down to brass) and then relocated the wire hoop to the other bottom screw (with a little lock washer under the screw head). This alternate screw location is not the lowest part of the mount and won't get any water on it.
HTH
