1992 ford ranger tail light not working?

1992 ford ranger, the rear running lights don't come on when i turn the headlights on. They use the same bulb as the brakes and the turn signals but both of those works, there's no voltage coming through the wire that is for the running lights when the switch is on, however the license plate lights both have 12.18V coming through and they light up. Could it be the wire has frayed over time? If not what else could it be?

Yes.

Check to see if there's trailer light wiring near the bumper. These are often temporary use adapters and often short out. BTW did you check if the tail light fuse is open