While I was away in nice warm Africa, my wife was stuck in ice-cold Canada, where temperatures were averaging about -20° or so the week before I came home. When I called my wife and told her I would be home on the weekend, she thought it might be wise to get the Phaeton washed, since she had not bothered to wash it since I left for Europe in November.
Very nice thought on the part of my wife, except, she didn't consider the effect of pouring lots of water all over a car that had chilled to 20° below the freezing point. After running the car through the touchless car wash, she drove it over to the gas pump to fill it up, and lo and behold, the gas flap was frozen shut. The kid that was pumping gas wisely declined to try forcing it open with a screwdriver.
My wife then drove the car home, and called our VW dealer. Mark, the Phaeton technician at our dealership, showed up at our home about an hour later, and he showed my wife how to release the gas cap flap manually, in case my wife ever decided, in the future, to once again turn the Phaeton into a large rolling Popsicle.
Well - my wife was pretty impressed with this level of service (and so was I), so one of the first things she mentioned when I arrived back home was "Guess what, I know how to do something with the car that YOU don't know how to do."
So - with thanks to my wife, and special thanks to Mark at my VW dealer, here's how you open a gas cap flap that is frozen shut:
1) Open the trunk, assuming of course that it is not also frozen shut.
2) Grab hold of either side of the little storage compartment on the right hand side of the trunk (the thing with the little net on it), and pull it straight out. It is held in place by spring clips.
3) If you are wearing any rings, watches, stuff like that, take them off now, because you are going to be sticking your hand into the area where the starter battery and a whole lot of electrical cables are found.
4) Look above the starter battery, on the inner fender wall, and you will see a white plastic component. This is the electrical actuator that pulls the cable to open the gas cap door. Stick your hand in there, and push the plunger in. "Pushing the plunger in", in this context, means you push the plunger aft and downwards, because it is installed pointing up at about a 45° angle.
Hopefully, the little door over the gas cap will now open. If it does not open, just push harder on the plunger.
Below are some photos to help explain the process.
After you have removed the storage bin on the right side of the trunk, this is what you will see:
Here's a close-up of the device that normally opens the gas cap door. The object in the foreground is the negative terminal of the starter battery.
This is what the device looks like when it is not actuated (meaning, the gas cap flap is locked closed).
This is how you want to squeeze it, to pull the cable and cause the flap to open.
NOTE: You DON'T have to remove it from the car, I only removed it to permit a better picture of it. I couldn't take the picture of the thing with the plunger compressed when it was still mounted in the car - my hand was in the way.