It is pretty easy. I put 4 lights into my 79 that had singles. From the donor you want the grill, the headlight trim, the light mount buckets, bulbs, and the wiring harness for both headlights. I took all the wiring from one light to the other, across the radiator core support, and into the other pair of lights. I wired it in to just one of the light sockets on the one side.
You can also re-wire so the lights are run from a relay right off the battery. Let the old light wiring trigger the relays to switch on the power direct from the battery. (with an inline fuse of course) The stock LUV lighting circuit runs full headlight power all the way in to the switch on the dash then back out to the bulbs. This drops the power to the bulbs by a considerable amount, and the switch can heat up and die as well.
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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