Link to Switch Image
I wired my MIM P for series/parallel switching and got beaten up by two Fender Luddites as a consequence.
For the record, I get two distinct tones from my P by flipping the switch. Series is the stock sound, and Parallel is thinner and crystaline sounding.
When I get around to installing my DiMarzio Model J pair into my MIM J fretless, I will also install series/parallel switching for each pickup, but not for the pair working together. DiMarzio provides 4-wire connections to support series/parallel switching. The Model J pickup is a pair of bi-lateral coils (E-A and D-G) that work as a hum cancelling pair, similar to the P pickup.