This is what solved my problem and what I posted last year under the thread: "Block heater question for '05 6.0". The solution was free and worked for me all winter (2009 - 2010) here in lots of wet weather so far without even one GFI trip.
I was prepared to replace the cord on my truck to fix a troublesome short that tripped the GFI in my garage last winter, but was able to fix the problem with the original cord and save some $$$. The dealer had wanted about $145 just for a cord, ouch. I asked at the International dealer and NAPA store, but could not get help there without a part number.
The factory cord is really two sections connected in the middle with a flat blade type plug. The end nearest the bumper with the male 115 VAC plug is approximately 18" long. The portion of the power cord containing the flat blade plug was secured with some plastic push pins and electrical tape to the front of the frame rail in front of the passenger wheel well. Unfortunately, the way it was hung there, that portion of the power cord was vertical with the female end of the blade plug facing up. In that position, it acted as a cup to collect whatever moisture intruded into the blade plug. I could not detect a short with my Ohm meter, but the small amount of moisture in there was enough to trip the GFI. The resistance between the hot and neutral plugs was in the range of 16 ohms, once the meter settled down. The resistance between the hot and ground and the neutral and ground plugs was either an OL (open line) or in the mega ohm range. The values seemed normal to me. I should recheck them now that the problem is fixed to see if they have changed and will edit my post with any change to the numbers.
I detached the 18 inch "pigtail" end of the power cord from the frame rail, dried out both ends of the blade plug with compressed air, used a toothpick to insert some dielectric grease into the female pins of the blade plug (not enough to cause oozing of the dielectric grease and more electrical shorting once the male pins were inserted), reconnected the blade plug and then wrapped the plug tightly with electrical tape to prevent moisture intrusion. I then used zip ties to rehang that portion of the power cord so that the female end pointed down. Because of the fixed length of the cord, I had to route it towards the center of the truck near the passenger side fog light before draping the end near the tow hook so that the blade plug could be hung female end down. There are probably several solutions to rehanging the cord. A loop could be one alternative, or the entire 18 could be left loose. I just wanted to ensure the blade plug hung female end down even though I had wrapped it well with tape.
The cord had tripped the GFI routinely before the fix, but has now been tested successfully several times, even as I wiggle the male 115 VAC plug to induce any hidden shorts in there caused by the corrosion that others have described.
It was a relief to go into the project confident with the help of some very informative posts and to find a cheap solution! Thank you.
2005 F-350: 6.0L PSD, XLT, CC, 8' Box, Auto/Tow Command, Manual 4x4, 3.73 LS, Camper Package, Snow Plow Package, Dual Alternators, Upfitter Switches, Clearance Lights, Reverse Aid Sensors, 12.5K Hitch, Skid Plates, Security Group, Medium Wedgewood Blue; Aftermarket: Retrax Cover, Line-X, WeatherTech Side Window & Bug Deflectors, JimmiJammers, Husky Mud Flaps.