Tried to post this earlier but I was having internet trouble.
I'm not sure this will help, but I found some wiring diagrams for a 2001 Golf. They're not the easiest thing to understand, but they might come in handy now or later. Search for 'heated rear window' and you'll find a switch, light, and heating element, nothing fancy.
My guesswork: Assume the following . . .
1) the element is working at over 12A (13.8A when the car's running)
2) and the car's electrical system is 12V (13.8V when the car's running)
3) The switch just puts the whole battery voltage into the element (very likely, as it's the cheapest)
. . . then there's less than an ohm of resistance in the circuit!
That does seem a bit odd, but consider that as the element warms up the resistance rises until it reaches a steady temperature, much like a light bulb. The easiest way for the fuse to blow is for the wire between the switch and the element to get shorted to the chassis (ground in a car). This would usually blow the fuse, but if it's a short near the back of the car then you've got that extra wire and connectors driving the resistance up (remember it only takes one ohm to make things behave weird), so it might not conduct enough current to completely blow the fuse. If it's very wet and dirty on the back window there might be a low resistance connection to ground that conducts after the element temperature rises. If this is the case then you've got two resistances in parallel, and the variable resistance in the element would make it self biasing, or if it was unstable perhaps even oscillating. How weird!
Most of that's fairly unlikely I guess, but it just shows how much odd stuff can happen with such a simple circuit. Here's what I'd do:
Contact a VW dealership (no matter where you bought it from) and see if there's a safety TSB for this issue. This assumes you're in the US, but there's probably something similar everywhere.
The NHTSA in the US has a search tool, and quick scan shows nothing directly related you'll want to check with a dealership to be sure. This means that it's a known issue related to safety, and they'll almost always repair the problem for free. Best of all worlds! Always check this before shelling out if you think it might be useful, it's saved me a lot of money in the past. Even if it's not a freebie, the techs there might be kind enough to answer a few questions that will help you fix it on your own if you ask nicely. I saved myself a few hundred dollars once by simply asking, "What the heck could even cause such a thing?" Good techies will problem solve a rhetorical question by reflex.
Check all the connections in the system, and if you can manage it all the wire, for signs of overheating, moisture, dried fluid, dirt, or shorting to ground. Sometimes the problem jumps right out at you, but even if it doesn't you'll feel better knowing it's probably 'not that'. It was probably a bad connection in the fuse (or maybe a bad fuse) as Jahonen said, but it doesn't hurt to check. I guess the real concern now is finding a replacement fuse panel, and my suggestion there would be a junkyard. You can check the other cars for signs of similar problems as you go, and maybe figure out more about your own troubles. I once pulled four vacuum distribution busses, which are simply plastic tubes with outlets of different sizes, before finding a good replacement. They'd all cracked in the same place, and further investigation showed that old motor mounts caused the motor to torque hard enough to yank on the short hose. In cold weather the plastic was brittle enough to crack. Two of the three bad ones I found in the junkyard came from cars that were in front end collisions (a big vacuum leak makes power brakes worthless, and stopping distance multiplies several times). If any of you have late 80s Ford Thunderbirds, be advised!
I hope that helps, I did ramble on a bit.
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