I could be way, WAY off base here, but I just remembered something..............The Ford Crown Victoria police cars have what they call "dark car" mode....the domelight is disabled, presumably so the cops can sneak in or out of their car unnoticed. I remember reading an article on www.crownvic.net about how to switch "dark car" on and off. You might get some help there.
Otherwise, this is how I'd start to troubleshoot the problem.
Unfasten the dome light so that you can A. Note the wire colors and B. test the wires. Hopefully have have a meter or test light, and know how to use it. In a pinch, you can remove the dome light bulb, attach wires to it, and use it as your test light.
When the domelight is on, one of the wires at the light should be negative, the other positive. I suppose that's obvious, but I'll get to my point........when the car door is closed, one of these wires gets switched off. Most vehicles use a "negative trigger" system, meaning the positive is constant at all times, and the ground is only present when the lamp is switched on.
For whatever reason, one of both of those wires isn't getting power. You should be able to follow those wires down the A-pillar and figure out where they go. Maybe someone cut one of them or something.
Also, you might try getting at the wires behind the headlight switch, and see if you can find and test the domelight control wire there.
Another idea: Make contact with the mechanics who work on the forest rangers' trucks. They may disable all of the trucks' dome lights in the same way, and therefore could tell you how to reconnect it.