I'm putting the 4 way switch in my Fender MIM tele and I'm having problems. The added position, which is both pups in series, is still giving me some hum. I thought that having them in series would make it hum canceling.

Is this not true if the pickups are not reverse wound? And further, are the Mexi tele's reverse wound?
What sort of diagram did you use for the wiring? Cause to make the humbucker position work on the 4-way switch you have to separate the neck pickups covers ground from the pickups normal ground to change the pickups phase.

It only cancels the hum if they are reversed wound. My telecaster doesn't have two different polarity pickups so I do have some hum in any position.

When you are in parallel with the pickups you will be a bit less susceptible to the hum even without reverse wound pickups. In series you will create a larger signal so the hum will be easier to notice.

I don't know the answer to if you already have a reverse wound pickup.