wiring a pj mustang series parallel

I had to think about this one for a sec but I think this should work. Take a look at the picture below for a series/parallel switch wiring. The two hot leads on each of your pickups currently go to two different lugs on the switch. Those are the red lines that come out of each pickup in the picture. Now two red lines that are connected to the center lug on each volume pot in the picture will go where your pickup hot leads were before the modification. That's it. I think that should work.

 

It reads like you only want to switch the split-coil (Precision) pickup from series/parallel.
In this case it would be easy to wire it up that way & include a pickup selector switch.

How many holes do you have for controls?

 

Use this diagram to have the blueprint for your series/parallel wiring. Not sure what the color codes are on the stock pickups, but I guess that's it pretty standard Fender, with white-hot, black-ground and some sort of wire that series connects the two coils. You are just unhooking the P pickup's wire from the 3 way switch, manipulating the wiring as shown below and then sending the new 'hot' to the 3-way switch.
Get a push/pull pot to do the work and send that black wire that is coming off the bottom right side tab on the push/pull switch to the appropriate tab on the 3-way switch.