Rabbits have fur coats, they're build for this. You can make them comfortable by giving them lots of hay/straw to snuggle in. Also, raise the cage off the floor (put it on a table or something). The cement floor of a garage gets cold and can cool the cage. You can also drape a blanket over the cage to keep heat in too.
They will be ok. The main thing to watch is that the water in their bottle and the nozzle of the bottle don't freeze. Rabbits can withstand cold, but they need to stay hydrated!