We've had the same problems with our 07 FEH; our guesses are similar: that the button gets bumped getting in/out of the vehicle, and/or the buttons are very sensitive and engage on their own with vibration.

Either way, it's really annoying. The 08+ switch placement is much better (though, on a separate issue, I now own an 08 MMH and the seat heater on the driver side only heats up once - the thermostat appears way too sensitive. There's a separate thread on that one, with no answers as of yet).

 

We have a 2007 Mercury Mariner Hybrid that does the same thing. It will switch on without even getting in or out of the car. It seems to always turn on when I am in the passenger's seat. It does switch on even when I carefully get into the car, but seems to do this more in the summer than winter. Has anyone found a fix for it? I have threatened to unplug the switch until winter.

 

I fixed mine. I did so by taking the set switch out of the link. I now have 2 LED rocker switches mounted in the center console. I ran wires (used a segment of 3 wire extension cord) from the wiring harness that plugs into the now unplugged switch at the seat under the carpet, up through the center console and onto the rocker switches. My wife, and by extension myself, is now a happy camper at least in this respect.