Heater always seems to be on

Probably have a bad Door Airflow control box

The door that mixes hot and cold air seems stuck in the cold air position. Check for a vacuum hose leak.

Isn't your heater supposed to blow hot air?

For this types of problem,i suggest you to check this help links:----Car heater blowing warm, but not hot air?http://schematicsdiagram.blogspot.com/2011/12/car-heater-blowing-warm-but-not-hot-air.html
---------A/C, heater, defroster fan, not working?http://technoanswers.blogspot.in/2012/02/ac-heater-defroster-fan-not-working.html
--------Heater problem?http://technoanswers.blogspot.in/2012/02/heater-problem-on-isuzu-trooper.html
---------Engine Overheats and heater blowing cold?http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html
-----------Heat blows cold on drivers side and hot on passenger side?http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html----------------This will help.Thanks.

When the coolant in the heater core is hot from the engine and your fan is on hot air will blow through the hot fan coil, i figured it out, when i stepped on the gas i could here the coolant squeezing by the control or limiter valve, because the higher rpms make the belt spin faster which makes the water pump pump harder causing more pressure, so when u idle that hot coolant will cool down from the fan blowing on it, u need to find the control valve and open it up to full blast, then hot coolant will always be flowing through your heater coil. ;)

Your heater valve is stuck open, also in a pinch you can crimp off your hose to the heater coil at fire wall. Try to find the heater valve vacumb hose under the hood see if it is bad and fixit. or some models have a cable to the valve could be broken. 

Everything that is heat and cooling in a vehicle is controlled by vacuum from the engine. It appears to me that your blend door vacuum line to the temp control switch is either off or cracked/broken and not making things work the way you want/expect. It's the vacuum friend.

The heater in your car depends upon a flow of hot coolant going through the heater core in order to get hot air into the car. If the heater core is partially plugged, it will restrict the flow. If you have a head gasket that is putting exhaust into the cooling system, it will eventually get into the heater core. When this happens there is no coolant in the heater core to make the air warm. If the water pump vanes have corroded away the result will be very poor coolant circulation causing the heater to not blow hot air. If your car is equipped with a heater control valve that is not functioning properly, the coolant flow will be restricted. One or a combination of these factors is likely the culprit in your case.
if the car is not overheating, the thermostat is working fine.... replace or flush the heater core multiple times to try and eliminate sediment build-up.

2000 jetta warms up when idling, but gets cold as soon as I start driving.

The heater blend door breaks on the 1997 to 1998 Explorers, you will need to replace the heater plenum case.