There is a nickel-chromium coil inside the switch that a contact rests on. When you turn the knob to adjust brightness, the contact rides on this coil. When I pulled my switch from my '66 (original switch), it was pretty cruddy but still functional. The contact was very worn out. Perhaps this is your problem. If your switch is original, you would be wise to replace it anyway. The internal circuit breakers tend to wear out with time and use, and your headlights can and will flicker as the worn out breakers cycle. While you have your switch out, you can run down your wiring from the switch with a test meter, and see if you can find your problem. Also, don't forget to check your fuse!
I replaced the headlight switch in my '68 and it produced a HUGE difference in my panel lights. They were working already but the change was at least 2X brighter. The switch is where I would start as it could be a quick and inexpensive solution (and like Mizzou said, it probably should be replaced anyway). If that doesn't work I would start tracing and checking wires. It's a pain but there's not much alternative.
Did your dash lights work before and just stop working? or did they never work? I bought my car w/o the dash lights working, changed the headlights switch, still didn't work. I pulled the dash panel out and ran a seperate hot wire to the lights connector to see if the dash was getting power, had to ground the gauge pod to test. Turned out someone had cut the wire that feeds the dash lights near the radio looking for a hot lead to run the aftermarket radio I'm guessing.
You may want to test the leads to the dash feed to make sure you've got juice after checking the fuses.
Check for corrosion on the connector from the harness to the inst. panel. There are two multi wire connectors. You want the one that has the blue/red stripe wire in it. I've seen these get pretty corroded from moisture over the years. This will also give you a place to check for input voltage to the cluster from the harness.
Check the panel ground. It's a black wire leading to a terminal that is bolted to the metal portion of the inst. cluster. It will get corroded from the elements also. This black ground wire leads to the same connector as the blue/red wire. You can check its connection with a Volt/Ohm meter set to the continuity setting.
1968 Mustang - I pulled out the panel, using a battery charger, I put 11.71 volts to the
pin that lights the panel. If I ground from the metal casing to the negative on the battery charger I get the panel lights with a meter reading of 10.73. ANY other method of grounding - no lights. I even tried grounding to the battery post, no luck. On the non-working grounding areas, the meter bounces around showing a negative 1 mostly. What gives?
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