I'm thinking its a fuse, as the PO left a box of fuses in the glove compartment.

None of the dash gauges light up.

I know it's not the switch as every other function on it (the cabin light, the headlights) work.

It's a '77 F-250

 

The dash lights work off the dimmer part when you turn the head light switch. Not sure right now if there is a fuse in that system other than the circuit breaker in the switch. Maybe the switch is bad or the little metal part that rides on the resistor is not touching?

 

The metal part that runs along the resistor often fatigues and fails to make contact, as well as its ability to corrode from sitting and rarely getting used.

 

Should be a tiny 3 amp fuse that goes to your instrument panel, you can try that. If it's a "new to you" truck, I'd replace the bulbs. There are black sockets on the back of your printed circuit board that you twist 90° and the bulb and all pops out. You can reach under the dash and do it without pulling the instrument panel.
X2 on the dimmer switch. Twist it left to right with headlights on and it should light up the dash if everything is in order.

 

Do you know which fuse it would be?

 

Top right on the fuse box.

 

It could be on the top right. Mine is the one before the bottom, and is so small you can barely see glass. If you look closely, your fuse box should say what they go to next to the fuse.

 

It is very common for the back of the cluster to start to crumble, & no longer hold the bulb socket securely. Then the bulb won't light, or the socket falls out completely.

If the back is solid, & the socket is there: Clean the contacts on the socket, replace the bulbs, clean the area on the PC where the bulb socket makes contact. You'll need to remove the cluster to clean the PC.

Sometimes the socket will look fine, but not light, replace the holder. I test 'em with a battery charger on the contacts before I install them.