Heating Problem

My 2000, manual transmission dodge neon (141,000 miles) has been having problems with the heating. If I turn it on, it will eventually get hot if the car is not moving. If it is cold outside and I am driving, however, the temp gauge will read cold, even when I have been driving for three hours. If I come to a stop, the temp gauge will eventually rise, only to fall once I get going again. This, of course, affects my heat. Whenever the gauge reads cold, my heat will never get hotter than about room temp. I thought of checking the thermostat, as it may be stuck open, but I have no idea where it is. Could that be the problem? And if so, where is my thermostat?