I had the common issue years ago with the defroster breaker tripping off after not even a minute so I did the classic fuse swap trick. Been working fine ever since. I don't use the defroster that much, but nice to have it actually work.

Well, recently my fuse box cover fell off (another common problem amirite) so I tossed it in the glovebox until I get around to fixing it. Something caught my eye a couple days later. WTH. Nice melty goodness. The defroster still worked just fine, and the terminals of the fusebox and the fuse itself are fine too. I guess the cheap fuse from the auto parts store was garbage plastic?

 

Both the fuse and the original circuit breaker are designed to pass somewhat more current than their rating before blowing (tripping). That will cause heat, which in this case seems to be more than the plastic can handle. The defroster (and seat) wiring is heavy enough to handle the high current so you really only have to worry about whether the fuse is accurately rated and would blow in case of a short circuit. It's unlikely that any fuse would fail to blow from a short, but I suppose it isn't impossible. The only way to check that is to test by increasing the current until it blows... destroying the fuse and making the test counterproductive. You could try moving the seats while the defroster is on to see if that amount of current draw blows the fuse. If you're concerned, perhaps picking up a well known brand replacement fuse might bring some piece of mind.

 

I have manual seats in my car, so the only time this fuse ever should see any current draw is when the defrost is on. And it was only used on occasion over the last 10 years, though this fall I've used it a few mornings in a row on my way to work. Since it was only this past week the cover fell off my fusebox I haven't looked in there until just now, so I have no clue when it melted (or if it melted slowly over multiple uses).

I'll grab a replacement fuse and see what happens. The fact the actual contacts haven't the slightest discoloration is reassuring as that is a good indicator that the wiring is perfectly fine. It is just bizarre, I've never seen such a thing, so I wondered if anyone else has seen this on their fuse swapped defrost.

Wish I knew what brand of fuse I bought from Advance 10 years ago. Not a clue at this point though!