Some home thermostats can 2 stage if they are made for AC and heat. You would need a 110v relay in the circuit, with a 12 volt coil. Should be around 15 bucks give or take. Radioshack would probly have one, the link above would work also, a socket mount for it would be helpful.
To answer your question, you would just replace the 12v fan with the coil side of the relay. When the thermostat closes it energizes the coil and pulls the contacts in on the relay, turning on whatever you have wired to the normally open side of the contacts on the relay. You could also run the coil and fan in parellel and still run the fan too if you wanted.
What are you using for 12 volt power source?
For the money for a relay, the fish tank controller above is a better option, IMO, unless you can get a relay for a really good price. I am ordering a couple of those.