I put a prewired PG with a set of Texmex PUs (SSS) on a Strat. Everthings cool but the bridge PU. It's tinny like stuck in the treble pos of a "wah wah" and the quack position sounds funny and hums. The mid and nk PU are fine.
The "hot" from the jack should go to the middle leg on the volume pot. The ground wire from the jack should go to the back of one of the pots, usually the volume pot, but this does not really matter as they should all be grounded.
I might some flack for this one but I had a strat that had weak tone and after resoldering everything a few times, replacing the pots & switch, adjusting the pups, and so on, over and over again, I found the problem was the tip of the jack pressing up against the body. And no, it wasn't shorting out on the shilding, the jack cavity wasn't sheilded. If I didn't know better, I'd have said the body was acting like a capacitor. Do you think candy apple red conducts electricity?
If the switch is wired to the center leg of the volume pot, wire the jack to other side, the leg that isn't grounded. Otherwise, what 1guitarslinger said.