Is there a trailer-hitch wire harness connector?
I'd be more interested in voltage-drop testing than resistance testing, but at least the wires are not broken and conduct SOME amount of amperage.
They're not supposed to be as bright as the brake lights. Verify the reflectors and lenses are clean. Assure the ground is good.
A 12-volt test light having an old-style incandescent bulb (NOT a new-style test light using LEDs) will give you a better idea of available voltage under load.
That's a puzzler. Apparently the switch works, but is somehow screwed-up in terms of supplying power.
The gear position (backup light) switch in my '88 was worn and defective, but it wasn't working "backwards" from how it should.
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Replacement switch, for comparison. Has an extra circuit that my truck doesn't use.
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Let us know when you figure this out.
1. Verify the bulbs. Some have two filaments, one filament can burn-out so the bulb still lights up "sometimes". If the third brake light works like it should...the brake switch is not the problem.
2. These vehicles have a known, common problem with the circuit board that holds the rear bulbs. Earliest trucks were worst--they used the old-style, push-and-turn bulb sockets. The later (updated) circuit boards use push-in bulbs but they still have problems with corrosion.
3. "Weird" lighting problems are sometimes due to a faulty turn signal switch. Look on your schematic to see which lights have the turn signal switch in common.