Thermostat location and replacement?

The heater works when the engine heats up. The problem is the engine wont heat up when its 32 degrees or less. I even have the radiator air flow blocked with cardboard and the engine still wont heat up! Thanks for your reply.

Thanks for the prompt reply zackman.I replaced the thermostat once with a generic and when it still would not heat up, I bought one from nissan for 20 bucks. Still no joy! This is a frustrating problem because of the seemingly simple answer.I am thinking about another tstat tho. Thanks again