I'm a bit confused. Your '58 should have already come with two tail lights + turn signals. Each of the tail lamps should be dual-filament (1157) bulbs with a dedicated line from the signals switch to each of the bulbs.

If the right rear is the only one that won't light, check for bad connections between the switch and the bulb. If you have continuity through that line but no power, it may be time to pull apart the signal switch. Inside the signal assembly is a curved sliding contact plate with raised contact points. All the signal wires come out of that and run to the bulbs. If the outside contact points are worn or misaligned the rear signals won't work.

I strongly recommend dedicated ground wires for each bulb; I soldered a wire to the outside of each socket and ran it to ground (on my panel it's the body, but the frame is a better choice). That chased off a few electrical gremlins and made further diagnostics much easier.

edit: "Washington State" is a big place. Maybe one of the 'bolters can help you with the diagnostic, if you care to be more specific.

1955 First Series GMC 250 1-Ton Panel
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Mr. Hedgehog - Follow the build
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