I'd have to look at mine to remember which is what, but I can tell you the rear four fuses are individual headlight fuses. You wouldn't see power on them without the headlights on.
You should check the wiring diagrams on the main
http://www.luvtruck.com information section. It doesn't label the fuses, but has them pictured as laid out in the fuse block and you can trace where they go.
As for getting a new one, most of the ones I've seen are in pretty good shape. Try looking at the junkyards and see what you come up with. I know 76 and 77 are identical, and I think 75 may be as well. In 78 the block changed (head lights switched to a thermal circuit breaker) so the cover is shorter. I've also seen one or two NOS ones on ebay, as well as used ones in good condition.
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95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good
