Admittedly I haven't tried mine yet, but here is a cheap inline thermostat
(120 V) that you could wire inside a fridge to a light bulb for a heat source. The temperature range is 50 - 90 F, which is perfect for primary fermentation. To lager, you will have to run the fridge (assuming your garage does not get below freezing).
If that temp range doesn't work for you, for years I used a Ranco single-stage temperature controller with a light bulb. If you don't mind spending around $60 for one, they are probably a better solution for you. (Here
is one also from McMaster-Carr, but you can find them cheaper elsewhere.)