As mentioned before, I had the exact same problem you experienced and the problem ended being mag chloride and road salts built up on my block heater chord connections; leading to current leakage. Once I cleaned those up, and sealed them to prevent future exposoure, my block heater is working fine in my GFI outlet -

Mark;

I haven't had a chance to check into that yet, but I don't think that it is the problem. The truck is still new (only about 1,500 miles on it) and we had such non-winter so far, it has had very little salt exposure, so I don't think that it would be a build up of that as the cause of the problem.

Thanks.