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I need more info. You say the dash lights go out when the headlamps are turned on? I am assuming you mean the dash lamps come on when you pull the switch out half-way, and then go out when you pull the headlamp switch out all the way?
I have no dash lights at all, with the headlamp switch pulled halfway or all the way,
However I do have Radio lights with the ignition on but as soon as I pull out the headlamp switch half or all the way I have no radio lights the radio still works I just cant see the time or station?
Also I forgot to mention its a XLT model if that helps
Here's a diagram. It's not for your year truck, but it's a good representation of what's going on.
You will see the lightblue/red feeds all the instrument lights. This should be true on your truck also.
I would check for 12 volts on one of the lightblue/red wires with a voltmeter or test light. Use a known good ground. If you get 12 volts, or the testlight lights, then find the ground that the instrument bulbs are using. See if you get anything there. If you don't then you know you have a bad ground somewhere like splice s201 or ground G103 in the diagram.
I had a problem with my dashb lights a while back too. Didn't matter if the switch was in parking lights or headlights. The entire dash was dark. The problem turned out to be a blown fuse in the fuse box. The fuse was the only 5 amp fuse in there. Fuse still looked good but my ohm meter confirmed it was dead.