Re: Replacement Fuse Box
I had the same problem and replaced the fusebox, best place is either a dealer if you have one locally, or E.B. Spares if not. Price is about £92 + vat, I'm sure they were cheaper (£70-80ish) just a few weeks ago, but there you go.
I've had the new box in a few days now and used the rear heater several times, and am happy to report that it seems to have worked - the rear window demists, as do the mirrors, and the fuse doesn't get hot at all! Of course, it could decide to start melting next week, next month, or next year, but for now its working and that's as good as I could have hoped for.