The best way I have found to heat my garage is to run a few errands in my car, then park it in the garage, close the door, and come back in 30 minutes. That engine block, radiator and two turbochargers do a great job of warming up that space.
If I want it even warmer, all I have to do is make those two turbos work harder while running those errands.
Of course, when it is really cold outside, the two intercoolers make the two turbos all the more tempting to work harder.
CD