Brought the answer from there over here:
I realize this is an old thread, but does anyone know what the white plastic part does that is inside the tail cap and is around the spring?
I'm pretty sure the white plastic is an insulator, and keeps the silver ring from contacting the end/wall of the body until you screw it down, completing the circuit to turn it on. The light body contacts the pos. end of the battery (through the dropin), and the tail-end center spring contacts the neg. end. You complete the circuit when pushing the switch down or screwing it down. At least that's how the Surefires work, just arranged a little differently.
I assume the switch is broken (a short?), but that's assuming you've made the light work with a different tail cap. If it won't work with any tailcap, then it's somewhere else in the circuit.