What types of plants? Tropicals? Subtropicals?

What are the temps in your garage? Do you have light in there?

I winter over a host of trops and subtrops with a thermostatically controlled electric heater and a fan to keep the air moving. I have two south windows and a skylight to supply a bit of light, plus two florescent lamps on a timer. I am able to keep the temps just above 45 on the coldest nights. Warm enough to winter over subtrop palms, bananas, brugs, citrus and epiphyllums. Not quite warm enough for plumerias and other really tender things.

It will really depend on what you are trying to winter over.