The phase 1 GTV's and some early phase 2's have a problem here, where the fuse (in the fusebox under the drivers dash), builds up heat, due to current load on it.
This melts the flimsy plastic fuse holder (& in some extreme cases can cause fires).
There should also be a multi pin connector under the glove box which carries the Br/Wh wire (supply feed back to the element on the rear screen), which also gets very hot and melts.
You need to cure these two areas first, i.e. a new fuse box in most cases and an inline soldered / crimped connection behind the glove box, (bypassing the multiplug).
The next step is to prevent this from happening again. This is a very simple fix and if you're handy with some basic electrics, won't cost much more than £5 for parts.
You need to remove the current load on the cable running through the car and use that to switch a new rear mounted relay in the back, as close to the window element as possible.
I'll post up a quick guide with some photo's when i'm back tonight!