I know this is an old post, but has anyone else had this issue? I just started having the same problem, except that it's the opposite. The cooled seat button activates the heat, while the heat button does nothing.
Drove the GT from Atlanta to Lansing, MI for the holidays. Started the trip with 420 miles on the car. I'm sitting at 1466 today.
So far, my MPG is around 25 for the trip with most of the highway miles giving me over 26. All tanks have been 93 octane.