This is standard Mustang wiring, view from under the pickguard, neck at left, bridge at right. The connections we will be modifying are highlighted in purple.
This is what we want to get to, the red and green wires are the new connections.
It is not necessary to completely disconnect the "purple" wires from the first drawing. In fact, much easier is to simply desolder the wires in two of the points (3 connections total to be desoldered). When putting it back together, in one case you can simply solder two of the short wires together and cover with electrical tape (which is actually what I had done), for the remaining wire you can solder a small wire (it needs to be longer) and cover it with tape as before.
The way this works is:
Switches towards the bridge work as normal (on for each pickup)
Switches work normal in the middle (off for each pickup)
You have to have both switches towards the neck to activate series mode. Just one switch towards the neck doesn't do anything.
You don't get an out of phase option anymore.
You should be able to mirror the diagram and have the series switches towards the bridge and the parallel towards the neck if you want. It has occurred to me that in fact the sound is not the same on a stock Mustang in the two outer positions, even on one pickup by itself (or both in phase), really, try it if you don't believe me. So according to this you may choose to keep one side or the other for the stock parallel connection.
Anyways, I wired my Mustang back to stock, but here is a video of the mod:
[youtube][/youtube]