Question: "I am using the bare spot on the bumper brackets (where I had removed the old tail ight brackets by grinding) as my ground." What exactly is "my ground"?
The test described by Phil is to take a voltage tester (meter or light) ant touch one probe to an absolutely sure ground (which could even be a wire all the way from battery negative post) to the contact inside the bulb socket after you take the bulb out. Then we can go from there, one step at a time. Nothing is for sure until you start testing as we describe. As we would do with our own truck. Report your findings.
Many of us do not use the "25 things it could be" approach. Most of the time we use testing.
Check for voltage with a ice pick tester at the bulb contact, pedal depressed, using a dedicated good known ground. Check both tail lights. Check both bulbs, both filaments, by taking bulbs out and checking them at the battery or any 6V battery. Voltage is the easy part of the circuit to test. Ground is much harder. That's next.
