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The instrument lights should be on fuse 12.
13.0 Electrical
Have you seen Clark's Garage? Clark's Garage Service manual has a lot of valuable and useful stuff.
Clark's Garage Home Page
As to the other fuse, you say it doesn't blow, but rather "disintegrates?"
The times I've seen that on my 83 NA (with the cooling fan fuse - #3), it was because the fuse circuit was overheating under heavy use.
In my car, the rear window defogger fuse (#4) had badly over heated at some time and partially melted the fuse box. My guess is that affected the other fuses, because they all got hot under heavy use (the closer to #4, the hotter). I was measuring temps as high as 185F - and sometimes (when the heater blower and the radiator cooling fans were all going) you could smell buring insulation.
The #3 fuse would get so hot the plastic fuse would melt (kinda like yours) - but it wouldn't blow. In the pic below, you can see how charred #4 holder is. You can also see how the plastic of #5 is looking kinda melted/charred.
I fixed it by re-wiring the aux fuse box - I cut the wires from the box and put new modern fuses in-line (same amperage as before). Problem solved. No more over heating wires.
If you could find the same sort of fuse holders/connectors, then you could rebuild the aux box. The connector/holder is one piece. I looked but was unable to find anyone selling them.
The problem I think was caused by the crimp connectors on the aux fuse box fuse holders. I believe that when they got hot (by #4 over heating) that damaged the adjoining connecors.