68 Barracuda Instrument lights not working

Check the electromechanical voltage regulator for the instruments, it's probably signed off this mortal coil.

Solid state replacements are available, and work MUCH better.

You know what? Ignore this post. I'm tarded and missed that it was the lights gone dead, not the instruments.

What MVH said.
The IVR has nothing to do with the instrument lights. It's one pin on the panel that gets its voltage from the headlight switch. It should be easy to trace back.