I bought my son a '99 XLT Extended cab a couple of months ago. I noticed the temp gauge wasn't climbing very highso naturally I assumed it needed a new thermostat. We replaced the thermostat, it seemed ok, was running a little warmer according to the gauge. We went on a trip this weekend from Cincy to Columbus and I noticed the temp gauge was reading extremely low again. My son drove it around town this morning, seemed OK. On the way back to Cincy I noticed the gauge showing a low temp reading again so I decided to turn the heater on high to check things. Nothing but cold air coming out the vents! If the water pump was bad I can see getting cold air, but the thing would be overheating. What gives?

JA

 

I know its a different car but the concept is the same. My brothers cavalier did this and the heater core was going out on him,(clogged) If you can find the heater core make sure both hoses are warm going into the core, one is inlet and one is outlet of course. I believe the core is going to be on the passenger side under the dash kinda by the radio. hope this helps

 

Agree with checking the heater hoses.Note that there is usually a valve in one of the hoses that opens when the controls call for heat.
Also do a search on the forums for Blend Door Actuator. These have been known to fail and there are solutions with pics available.
As for the normal / low temp readings , this could be due to either a failing temp sensor , low coolant (an air bubble in the top end not letting coolant flow past the sensor properly) or a stuck fan clutch.
Check for low coolant by parking the truck facing uphill until cool and make sure the radiator (not reservoir) is full. You may have to check this a couple times to be sure.