Make sure you insulate and get it off of that cold floor somehow, maybe put the box on soda crates or something similar. I would suggest that you have your light cycle be on at night to alleviate some of the low temp problems. You will still probably need a heater of some sort, just make sure that you have plenty of circulation so that the heater doesn't create hot spots while leaving some of your plants cold. Also you could try not ventilating during lights off; your plants don't need as much co2 at night so this shouldn't (in theory) create too much problem. If you are ventilating when your outside (garage) air is 5 degrees it will be almost impossible to keep your temps up. Good luck.