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.

Your thermostat is probably stuck. It is only like $5 for a thermostat, replace it and see it that fixes it.

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