Hey there...
If you raise the rear of the car you will see one clip with a wire coming out of it. They are quite difficult to take off, but basically on the bottom I believe there are two teeth that grip the plug. I usually take a flat-head and pry it in there and slide it off. It takes some playing around with as it is a very tight/snug fit.
Once you pop it off, I usually spray it really well with electrical contact cleaner (both ends). Once that is done, I use dielectric grease and smother it on both ends and plug it back together. I then take the dielectric grease and smear it on the outside of the plug as well, somewhat plugging up the holes and stuff that may let salt water reach the electrical contact surfaces.
Once I did this to both rear wheels, never had it come back again. Every fall I re-clean and apply the grease as a preventative.
