'66 mustang fuse panel ?

Firstly, get wiring diagram, then it's easy to use multimeter to check where voltage stops coming.

use a good quality srpay electrical contact cleaner, fist pull all the fuses then spray the contact use a small brass brush to make sure the corrison is loosened. The power for the instrument lights come from the headlight switch on a blue wire with a red stripe. this then goes tot fues panel intrument lamp fuse then returns to the cluster on a blue wire with red stripe. also make sure the cluster has a good ground. sometimes when a dash board is painted the paint coats the inside of the screw holes preventing electrical contact. If the power is not coming from the headlight switch is may be the dimmer (large coil of wire inside of the porcelin wheel) these are prone to poor contact breakage and corrosion. A new headlight switch is about $25.00 from most dealers and very easy to change if you have thin wrist (haha)

no power to the guages indicates a bad voltage regulator on the back of the panel. It's about the size of a 9 volt battery in a silver case (maybe rusty now)

If you check it with a voltmeter, it should have =12 volts at one terminal with key on and +5 volts on the other terminal feeding to the guages.

My guess is you don't have the +5 volts, hence a bad regulator.

Most mustang parts suppliers have a replacement.

Curtis