Stoyce - Excellent post and guide
Talking gonads I think. There's still scope for it still going horribly wrong if the wrong gauge wire is used. But the wire run under load is a lot shorter and what there is, is probably better able to dissipate heat. The load across the original fuseway has been dramatically reduced to a few milliamps for the relay coil and a few amps for the door mirrors.
I'd like to hear his reasoning in detail
I totally agree.
These sorts of problems have cropped up many times on other cars. It's naive to expect manufacturers to issue recalls of cars over ten years old... The X1/9, for example, had a horrible problem with the main supply wire (un-fused) to the fuse box, which had a big hot connector floating around under the dash. If you were lucky, the only symptom was that everything electrical worked poorly and there was a burning smell. If you were less lucky, the wire melted the insulation from that connector all the way forward to the battery, or (if even less lucky) the car caught fire. The trick was to get there first and add a thicker supply wire, well-sheathed and run through grommets, with the troublesome connectors bypassed.
I think the melting problem at the GTV fuse box might be caused by the fuses in this type of fuse box not really making a very good connection with the terminals (and it gets worse with age and dampness). It's OK for small loads but not for 30A. Obviously the connectors in the wire as it makes its way through the car are also a problem. The relay modification solves both problems and without a doubt is worth doing.
I urge anyone who doesn't feel able to carry out the relay modification to at least pull the fuses one-by-one and clean with contact cleaner, maybe even replace the fuses as a precaution (and use name-brand replacements, not the cheapest type available). Prevention of a melted fuse box is so much easier and cheaper than cure. Even if the fuses are starting to melt, cleaning contacts usually allows a new fuse to be installed safely.
No doubt about it, placing a relay near the load isn't a bodge, it's good practice. For example, headlight relays mounted near the headlights are a good idea. The 164 has those (annoyingly only for main beam as I recall, I spend most of the time on dipped beam).
The 164 has its heated rear window relay in the boot, near the battery. Ideal, then, except for one flaw - it had the same poor-quality type of blade fuse holder and 30A fuse as the fuse box at the front of the car. Therefore the fuse would still get hot and the plastic would melt off due to the poor connection to the fuse. But at least the rest of the wiring and the main fuse box was safe!
So - I suggest when making this modification, buy the best quality fuse holder that you can find
Ideally I reckon one of those bolted-down fusible link types, like newer cars (156 etc) have in a box on the battery terminal. It would, of course, need to be covered and fastened to avoid any possible contact with metalwork.
The type of garish gold-plated fuse holder used for fancy car audio installs would also be OK, if maybe a bit over the top. What you don't want is something rated at a maximum of 30A, as I've found that these are often not suitable for a sustained 20-30A. The terminals need to make a really firm contact with the fuse...