These are typical symptoms of a bad ground wire at the rear. As a test to see if I am right, turn on the right signal and watch it flash rapidly. While that is occurring, remove the bulb on the left side. If doing so makes the right one go off completely, that is the way the circuit should be diagnosed; (with the left bulb removed and the right circuit bad).
With a broken ground, current going through the right bulb cannot find a return path to ground, but it finds an alternate path by doubling back through the tail light filament, then over to the interconnected tail light on the left side, then to ground over there. You actually have three filaments in series. Both on the right side light up dimly so in total they look normal brightness, and if you look closely at the left bulb, you may see it glowing very dimly.
Since right signal current found its way onto the tail lamp circuit, that circuit feeds the dash light circuit back at the head light switch. That is why the dash lights come on too.
The back-up lamps share the same ground connection with the tail/signal lights so they are in this story too. You may need to also remove the left or both back-up bulbs to see the symptoms change. You can also verify a bad ground with a voltmeter. With just one function turned on, go to the right rear bulbs and probe each terminal in the sockets. In both sockets you must find one wire/terminal that has 0 volts on it all the time. When you have a bad ground, you'll find the normal full 12 volts on one wire, and between about 4 to 6 volts on the other wires.
Being a truck, it is highly likely someone added a trailer hitch and trailer wiring years ago. It is common to splice in the wires with Scotch Lok connectors that just snap together. Those should never be used outside the vehicle because they do not seal out moisture. It is real common to find the truck's original wires corroded away at those splices.
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Sunday, August 12th, 2018 AT 10:07 PM
