Gav is on the money with his info. One thing to note is it requires a CRL style lever switch to make it work, as you need a separate 'common' for each pole on the switch and the stock PBG 3-way 'import' style switch usually just has a single shared common lug. And, if you need to change switches, the alternative is to just use a 4-way lever switch so you have both series and parallel options without the need for a separate mini-toggle.

Here's some info on how to do the parallel/series wiring mod to a stock PBG Tele neck pickup.