2000 Jetta Heater Not working

Sounds like the thermostat needs replacing... should cure the issue...

Leajking hjead gasket most likely.

check your coolant in the radiator and for coolant leak. feel the heater hoses that go to and from the heater core to see if both got hot. if one of them feel colder than the other then there is a blockage in the heater core.

Does the fan blow air, but it's not hot?

For a heater to operate properly, hot coolant must enter the heater core inside the car. A valve opens and closes to allow this coolant to flow and to stop it when heat is not needed. This valve is usually located close to the firewall at the back of the engine. If you had work done on the hoses, it is possible that the connections to this valve were not done properly.

If you can locate this valve, watch its operation when someone in the car turns the heat on. There is likely a cable which runs from the heating system to the valve. This cable will change the position of a lever on the outside of the valve which will either open the valve for the hot coolant to come inside or shut the coolant off. There should be at least two coolant hoses connected to this valve. This cable can get detached from the lever, and this will prevent the flow of hot coolant from coming into the heater core.

your heat is not working becuase the coolant level is too low meaning your heater core is full of air. the reason your car overheated is because the water pump impeller splines are gone and you need to replace the water pump. it is driven by the timing belt so replace that too. and the tensioner. this is the number one cause of vw overheating, especially on a diesel. i have replaced about 50 water pumps on this engine because of this.

head gasket.

You've lost engine coolant somewhere, and need to fill the engine back up with the VW special G12 coolant.

Then find the leak, which will be in one or more of the following places -
Water hoses to heater core, including any other coolant carrying hoses in the engine compartment
Radiator hoses
Plastic manifold bolted to side of engine which is known to leak (have replaced on an '01 VW Beetle, and an '02 Jetta, both members of our family in the past few years.).
Overflow reservoir cracked
Radiator leak.

Reply back what you find. If the engine got too hot, you may need to have the thermostat replaced.

I do believe soem 2000 Jetta TDI's came with block heaters. Normally a heater of this type you plug in is to keep the oil warm. This will not keep the block very warm and will not decrease the amount of time it takes to heat up.

If you want the car and the heater to get warmer quicker, you should invest in a "Zerostart coolant heater". There are company's that sell these with all of the hoses, bolts, and clamps needed for a quick installation. If you can change brake pads, you can install this.

I currently have this installed on my TDI and have it plugged into a timer. It comes on about 2 hours before I leave in the morning. It electrically heats the coolant and thermosiphons the coolant through the block. I am now able to immediately begin defrosting the windshield at start up.

plastic impeller on water pump has come adrift

U'll need to bleed the system.