weak heat

2001 bravada 4.3 vortec. it just got antarctic in my town and now i go to turn the heat on and my car does not get as hot as it used to. ive also noticed that the temp gaugebarely moves past 100 degrees even after driving all day. i have turned the heat all the way up but it just gets warm. i think it might be the thermostat stuck open or maybe something to do with the antifreeze. i hope my heater core isnt out i dont want to be out in the cold fixing it. as always any and all info greatly appreciated. thanks in advanced guys. :confused:

Yup, your thermostat could be stuck open a tiny bit.
With the engine running and as warm as it'll get, feel the upper radiator hose at the radiator.
If it's warm, the thermostat is leaking by. If it's cold, your heater core is drawing out all the available heat, acting as the engine's radiator.

You know it yourself, Allegretto, it's most likely a bad thermostat. A $10 dollar part and easy to replace, but don't forget to get a new radiator cap at the same time. They have about the same life expectancy.