Is the garage attached? If yes, why not look into a proper garage heat source fed from the house (probably natural gas fed garage furnace?)
Why not at least insulate it if you're going to do a lot of winter work out there?
my garage is drywalled, attached - the ceiling is drywalled, and uninsulated. I don't believe that there is insulation under the exterior walls' drywall because the builder was THAT cheap.
that being said, the garage stays warm enough that anything stored adjacent to any of the common walls never freezes, ice/snow that may have accumulated on the cars during the commute always melt off over night, and I get by with a simple 20lb propane bottle-top single burner unit that was something like $24.95 at home depot for the very rare times I am forced to do anything more than put washer fluid in the car during winter months.
Hopefully I didn't just jynx myself and have to do something more serious before warmer weather...