Did you get a chance to see if that bolt on the block passenger side or
firewall side if front wheel drive with the small black wires off it was
still in one piece with no broken wires?  That is the computer harness
ground.

Headlights run off of two things though.  Power and ground.  You just
verified the ground, now the power path is suspect.

Your switch is ok.

The power for the headlights and the other wiring harnesses in the
vehicle come off the starter solenoid or the power distribution box.
Lets go for the headlight one which for sure comes off the starter
solenoid.  Where the main battery cable bolts onto the solenoid is a
post with a few small fat wires called fuse links that are about 8" long
and join the wiring harness with big fat rubber crimp connectors.

I would clean this post and all the loop connectors hooking onto it to
see what happens then.   You might have to remove a plastic cover to get
at them, they are on the end of the starter.

Could be all the intermittent things are coming from this one dirty
post.

Just another guess, hope it helps.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's