#1
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?
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#2
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.

http://www.seymourduncan.com/support/wiring-diagrams/schematics.php?schematic=changing_tele_lead_phase
#5
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.