So far you have proven that you have 12V to the socket. You have proven the bumper has continuity to the battery neg. You have NOT proven the socket housing/"chassis" is grounded to the battery and you have not said you have tested the bulbs right onto the battery. (Touch bulb terminal onto battery pos and touch the bulb base to battery neg to test) If the bulbs are good and you have 12V to the socket terminal and the socket housing has continuity to the battery neg, the light will come on. Hook a test wire from the battery GROUND terminal and touch it to the housing of the socket. The housing is the part that the bulb goes into. This housing has an insulator with two bulb terminals, as you know. The body of the bulb is ground and touches the socket housing at the bayonets, the socket housing is grounded to the grille or whatever metal it is mounted to. (don't touch this test wire to the terminals). You have a ground problem, that's all. Every major component on a truck is "isolated" by rubber. The engine, the body, the frame. You need battery ground to engine, ground strap from engine to body, ground strap from body to frame. Good dash ground for dash bulbs and gauges. Various short ground wires at sockets, dash lights, turn sig bulbs, gas tank sender. Also clean and sand where lamp sockets touch bezels and grilles and fenders and body panels and all these must be to bare metal (no paint, no rust, no grease, no corrosion). A wiring job is no good and you will chase these intermittent problems over and over if you do not ground everything carefully. Then your frustrations will be over.
