sounds like the same thing i've gone through.
try getting a new dash switch, i don't think it's really the problem. i've just noticed changing the switch works (for a while anyway) the 84 has gone through 3-4 switches, 93 has had 2-3, and the 91 (newest truck) is on it's 2nd one.
i've had this problem with all my truck basicly and have asked on here quite a few times and never got any real answer. so i just change them out when i start having trouble with them. i think the problem may be in the harness plug for the switch. i've noticed the terminal for the high beam (on the switch) always tends to look brownish around the base (shorting and overheating?) and seems to loosen up and get alot of play in it.
my 2cts,
Jon Boy
Jon Boy, having similar similar problems on three different trucks is extremely bad luck, and it's got to be just an unfortunate coincidence.
For what it's worth, like you I have a 1st generation, a 2nd gen and a 3rd gen. The 3rd gen truck, the '94, is the only one of my own trucks that has given me headlight problems. The combination switch kept cooking, so I finally converted the headlights to being powered by relays (I think there's a writeup in the tech library).
On your trucks, it sounds like like you've got an excessive load on the light circuit, to go through that many main switches. You're probably right, in saying it's in the wiring harness. Even if it's never been touched or modified, the wiring can degrade over time, especially in the wiring harness connectors.
If you have more lighting trouble, check the current draw, go over the wiring for corrosion, heat damage, etc., or consider just converting your lights over to being relay powered.