I wouldn't dare keep my chameleons in the garage...but then I live in Canada...so you can understand why! I do heat the room that they are in a bit in the winter time though...it gets colder than I would like it too during the night. I use one of those electric fire places...but I don't have the flames showing. Sorry I'm of no help on this.
I live in ca and I figure summer is coming soon. My room really started to smell bad and my parents started to worry about my lungs in the long run. I think the chams would get better air flow down there also.
I keep my chameleons in an insulated shop. I use two 240 v electric unit heaters to keep the shop ambient temps in range, and have a back up wood burning heat stove in case of power outage.
Putting the chams in the garage may work for the time being, but you really need to look at installing heat and/or air conditioning. Perhaps the best solution would be to keep your chameleon's cage clean. I agree with Marc, your cage shouldn't stink.