I'm having the same problem. Can you explain the process of working backwards. I know how to do things if explained but I have no experience working on electrical problems.
My 1998 Ram 1500 has had problems with no tailights and no interior lights before. I changed the headlight switch and checked/replaced fuses under my knee and in the engine. Kept blowing a fuse under my knee until I dimmed the instrument lights.
Recently, I smelled burning plastic coming from under the dash near the headlight switch and shortly after the police stopped me for no tailights. They haven't worked since.
Checked tail light (running light) bulbs and switched them with the brake lights. Bulbs good. Used a pencil tester to check fuses. Good. Disassembled tailight assm and found frayed wire on drivers side and splice/repaired it. Someone said I'd have to trace the wires from the tail lights to the fuse box, another suggested changing the wiring harness. One mechanic took a quick peek and said power steering fluid or oil was leaking on the wires and could have shorted them.
P.S. - I'm also getting a check engine light to replace the o2 sensor but don't know if it's related.