I lived in Minnesota for 10 years... there, it gets cold. We have, on occasion, found a lizard that managed to get into the cage that can't figure out how to get back out, but that just brings a smile.In my opinion, acclimation is the key. When we first got our birds (a little over a year ago), we only had them on the porch for 1-2 hours a day, and that was always in the morning or evening when it was still moderate. We slowly extended the time we kept them out there, getting into the mid-afternoon, heat-of-the-day times. We occasionally saw them panting, but they were never stressed or in danger of overheating or dehydration.
As someone pointed out, the natural habitat for most captive birds (not just finches) is tropical or very warm. Some of them are from desert areas that have 100+ temps in the day and below 40 in the evening (F, not C). These are tough little birds... you just have to give them time to adjust to the environment in a non-stressful way.
As for the garage... air circulation would be my main concern. While it may get pretty warm in there, I'd be more concerned with the stagnant air. I know I wouldn't want to breath it for very long...
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