I have a 99 XJ that lost the front running lights, dash illumination, and tail lights.
I checked the two fuses in the fuse panel inside the vehicle (#7 and #23 for the 99 XJ), they were good.
Using my VOM and a wiring diagram, I checked for output voltage (blk/yel wire, I think for the 99) at the H/L switch: 0 volts. Ah-HA!
I replaced the H/L switch with a known good one, no change.
Referring once again to my wiring diagram, I checked for voltage coming INTO to H/L switch for the lights that are out (tan/? wires, for the 99):
0 volts. Hmm...
....back to the wiring diagram. What's ahead of the H/L switch that feeds current through the tan/? wires?
It seems there is a mysterious "fuse #17, 20 amp" before the H/L switch.
Interior fuse box has a #17 fuse, but it is clearly NOT for the dead lights, according to my wiring diagram.
I check the "main" fuse box under the hood. No numbering there just the goofy list inside the lid, and some of the fuses aren't identified at all.
So I checked every 20 amp fuse I saw there, and voila! I found a blown 20 amp fuse!
I replaced it with a spare, and everything is back to normal!