If the radio light works AND dims when you turn the knob then that rules out the wiring and headlight switch
The only things left are all the bulbs are burnt out / not making good connection, the wire that feeds the instrument panel lights is damaged or it's connections are not making a good connection, or the ground to the instrument panel is gone.
If the ground to the instrument panel were gone the temp, fuel, oil pressure ( if equiped) gauges would be dead or pegged on high. ( The IVR must have a ground to function and if it does not have one it supplies full battery voltage to the gauges)
The reason the gauges might be dead is they were pegged high so long they burnt out.
I would start by getting a wiring diagram (
mymopay.com) might have one free download or get one from
classiccarwiring.com ( best 25.00 I spent.
Then trace out which wire (pink? Orange?) feeds the instrument panel lights.
Then pull the front seat/s and get under there and look for voltage where it's supposed to be