If you are not using it in a building, where are you using it, and what do you want to measure the temperature of? Air, fluid, or something else?
If it is air temperature, you probably can just use a cheap line voltage thermostat to switch the 12 vdc directly. No need for a solenoid or relay.
Take a look at
http://www.google.com/products/cata...og_result&ct=result&resnum=2&ved=0CH4Q8wIwAQ#
These things are normally for switching the power to high current AC resistance heaters. It won't be rated for 12 volt operation, but I can't think of any reason it wouldn't work, unless you are using some sort of monster fan motor. It should be nothing more than a set of heavy duty adjustable temperature sensitive bi-metallic contacts.
I had a house with a similar thermostat switching the fan motor on and off on a hot water radiator in a garage. Worked fine.