Since both running lights are on the same wire it has to be a wire problem but to be on the safe side, start by chkg the bulbs, then the sockets and make sure they are in good order and clean.
After that start tracing the wire forward toward the cab and see if there are any pinched or broken areas in the wire.
Not totally sure on the 90 Ranger but if all is OK from the lights to the cab, there has to be an interface that is having a problem.
I had basically with a ’95 Ranger XL, and it ended up being the Multifunction Sw inside of the steering column, part of the directional system sw.
Have you hit anything under the truck lately, changed anything or added anything ?
All things that may have changed or pinched a wire.
When you are looking under the truck chk for a broken body mount, may have pinched a wire.
Just thinking out loud so luck to ya.
4x4, 5sp, Std Cab, Stepside, E4 Red
4” Lift - Coil-Overs, 6” SkyJacker Leafs - No Blocks (Mistake)
31 Spline FX4 Rear Axle, 4.56s and OX Locker, Hurst Shifter w/Optional Stick and "Made by Grev" alignment adapter