I don't have heated seats. But what is stopping you all from cutting and splicing a "dimmer switch" from your local hardware store?

Check the AMP draw ( volts is a non-issue ) of the car seat heater, and get a dimmer switch for your car that can take it. Put the FEH on "high" and leave it there, and turn the dimmer down to your perfect liking.

Who can tell me why that won't work?
( like I said, I don't have one, but if I did, I would at least try that )
-John