Phase Reversal Switch

The input on the diagram(toprow of the poles) is where the signal (from your P90) comes in...
"+" Usually denotes the "hot" or signalwire, "-" is the ground.

The middle row of poles attach to where the pickup normally would go. One wire to the lug of the volumepot. One wire to the back of the pot.

Ignore the blue dotted lines. Those are the internal connections being made.

As long as you distinguish between the top and middle row it doesn't matter whether you mix up your plus and minuses...

Good luck!

 

Single core + braid is gonna be iffy on a phase switch. You turn that braid into your hot lead, that's begging for noise. I don't know about you, but I dislike re-wiring pickups unnecessarily; tends to be delicate business. If you're confident doing it or know somebody who is, it shouldn't be hard to rewire one of those P90s with two core. Phase switch needs to happen to one pickup before volume, tone or selector; the other remains wired as normal.

One of the best resources I've ever seen for this sort of stuff is Adrian Legg's Customizing Your Electric Guitar. Great book with lots of ideas, well written and easy to follow even with no electronics background.

-m