1992 no headlights until it heats up

Trying to help someone with a 92 K1500 this weekend. The problem is that the head lights will not work until the truck heats up. I have limited info at this time, but being told that once it heats up, they come on. So this lends me to believe that the problem is inside the truck some where. I am thinking thermal expansion, when the wire gets hot the short completes and the circuit? He said that he had this issue a few years ago and replaced the switch but that did not fix the issue. Any ideas of where to start? I am still thinking about starting with the switch. Maybe start with a multi meter and keep looking for where there is power? Since they are both out, start looking for the issue where the power is common for both R and L. Is there any place that i can find a wiring diagram?

try all the connections in the circut, where the wires hook up to the bulb, the battery, and anything else that completes the circut. Also make sure the wires arent corroded on the inside.

Is it ONLY the headlights that don't work or headlights and parking/corner lights?

It may be a sticking relay as well.

The parking lights are affected too. The parking lights will come on after 5 min of driving but the head lights will not come on for 30 min. It varies of course. It does not act like a typical intermitent short. Any ideas?

What relay is in the lighting circuit?

While trying to fix this issue a while back, the switch was replaced and it did not help. I have not seen it yet first hand, and will still check the switch.

i have a 93 gmc with the same problem i found that if you take off the gray pannel where the switch is and make sure the connetions are still together and wigle the switch they come on. at least that is what my truck does.

I think it is the switch...test with meter to verify..

on a 1992 I don't know if relays were used...if so then relay failure is possible ..

since it is a repeating failure it most likely is one of these items rather than lamp sockets corroded or grounds "BUT" I would still give those a look before spending any money ...