Hm... good luck wedging anything other than a Volvo heater fan into that little space behind the dashboard. The motor is certainly not the world's biggest, but it isn't too bad. They do push a lot of air around.

There's a different way to approach this, though. You could put an outlet fan in the back, under the floor/inside the right side rear panel where the little air duct goes to the outside. Naturally you'd have to modify it, but it would be quieter back there and would heat the entire car because it would draw the heat through from behind. That's just another one of my wacky ideas...