You could try wiring them in series with a switch. That would reduce the temperature by half. I think seat-warmers are overdue for a variable setting. I don't use the damn things, they make my rear itchy.
My UK PiP has high and low settings for the seats but I have no evidence that there is a thermostat, just gets hotter and hotter.
The 2012 PIP has a switch that sets them to hi or low. You could check into what that switch does and see if it would work in your car.
Yes they def do get very hot very quick…..You would have to have an on / off battle with the button when its cold out..... I test drove a 5 and yeah, sticky butt… lol weird how on a 30k+ car there is no adjustable settings? But the 5 does have the "wonderful" ruin your just washed car, "hood sprayer thing" aka headlight cleanser…haha, what a useless gadget, it actually turned me away from the 5 which I would have got a great deal on.. I have a Persona with softex (very nice material btw) and find so far that I don't really miss my Dodge Chargers heated seats, I do miss the heated steering wheel a little though on cold mornings.