Is it possible? Sure. I've stuck a couple of my seramas out in the garage, and I know many who will bring (outdoor) pet rabbits into the garage for the winter.
But I'd think it'd be a logistical nightmare if you had more than 2-3 birds. And you couldn't leave a car running (just driving in and shutting off the engine is no big deal - but letting a car warm up in a closed garage for ten minutes would probably put off a lot of fumes)...
In a two car garage, I personally wouldn't worry too much about ammonia build-up with just a couple of birds...assuming you keep things fairly clean, and I don't see you attempting DLM in a garage...lol!
I do baby my birds a bit, so I'm not going to say "don't do this or that" when it comes to worrying about making chickens more comfortable. But the fact of the matter is that in a draft free coop, with good food and liquid water available, healthy birds (at least winter hardy breeds) can deal with cold weather fairly well... Plug in dog water bowls are a cheap, simple way to keep water unfrozen.
