To pasteurize, I think the mead would have to at least get past 140°. Not only is that a monumental waste of energy (heating an entire garage for pasteurization), but it's probably not going to be good for other things you might have in the garage. Not to mention that exposing mead to such a high temp for such a long period probably won't be too good for it.
Either way, there's no way it's worth it. If you want to do something like it anyways, you'd probably be better off putting it in a car directly in the sun, with the windows up. You'll need a thermometer in there to make sure temps are sufficiently high. While you're at it, I strongly encourage you to take a picture of the thermometer and pm it (along with the date and location) to the HBT user by the name of marubozo - he's compiling important data on the matter. The same applies to anyone that wants to help him out by taking a picture of a car thermometer.