Tom, here's - surprise surprise - a Seymour Duncan diagram. It's for a Tele with two tapped pickups, so ignore the 3-way switch etc and just look at the push-pull pot.
I don't know how BKP wire the tapped P-90, but it'll either have 3 wires and a bare shield or (maybe) two wires and a braided shield.
Assuming it's the former, the shield and one wire (black?) will go to ground, the other two wires will go to the two outer lugs on one side of the push-pull. The middle lug of the push-pull then goes to the first lug on the volume pot (where the hot wire from a normal humbucker would go).
You should have full power with the switch down and tapped with the switch up.
http://www.seymourduncan.com/support/wiring-diagrams/schematics.php?schematic=tele_2_tapsThe problem is, I don't know the colour coding - on the Duncan diagram, black is ground, white is hot and red is the tap wire. BKP may be different - might be worth checking with Twinfan, I know he had a tapped P-90 or MQ.