Instrument lights not working

I think as long as it's a 12V source and you have a working gauge VR:Mustangs Unlimited - The Premier Source for Mustang, Shelby and Cougar Parts and Accessories
on the back of your gauge panel it should cut it down to the correct voltage for the gauges. The dash lights were run through the headlight switch stock.

If you don't have it already, here is the schematics for your car:
1966 Mustang Wiring and Vacuum Diagrams
print it and use it to trace your wires you will need and tap back into the headlight switch where needed. Also get a $5 test light if you don't have one to verify power at each wire you hook up or need power from.
Jon