Please help...how to keep pet rats cozy-pie in a garage?

I'm planning on keeping 2 pet rats in a cage in my garage and want advice on the best, economical way of keeping my ratties cozy. I can't spend a lot on heating, but want some method that will keep the garage warm-at the moment the garage is soooo cold and I've heard that there are various ways of keeping rats warm...heat lamps, ceramic-bulb thingies, halogen heating...but which is best??? Please help