I would NOT heat it, it's encased in plastic and it will melt the box behind the firewall then you will have to buy a whole new unit and pay someone big bucks to install it. Do you have a harbor freight near by? they seel special socket for this kind of situation that will take it off in the meantime soak it with wd40.
9 Piece 3/8" Drive Metric Bolt Extractor Sockets