2002 Volkswagen Passat coolant temperature sensor location

Depends on your engine's ID.

Digifont

There are two coolant temperature sensors mounted in the upper radiator hose-to-engine flange. The black connector is for the gauge on the instrument panel. The blue connector is for the ECU.

There is also an air temperature sensor at the inlet end of the air flow sensor. Both temperature sensors operate with the same resistance values.

Motronic System

The Engine Coolant Temperature (ECT) sensor is mounted in the water outlet on the back side of the head. If you vehicle still has a original VW part it will be black in color with a four terminal connector. This device signals the engine management system with the current coolant temperature, the ECU takes this information and uses it to control other such circuits as the idle speed, knock control system, oxygen sensor circuit, exhaust gas recalculation, and fuel tank venting. It is also important to note that if at any time ECU does not receive a signal from the ECT it will substitute a value of 176 °F (80 °C).

Vr6

The Engine Coolant Temperature sensor is mounted in the thermostat housing. The body of the sensor will be blue and the sensor will be blue and have
two terminals if it is an original factory VW part.

Allow the engine to cool below 100 °F (37 °C) before working on the cooling system.
Note the anti-theft code and radio presets for the radio.
Disconnect the negative battery cable.
Locate the Engine Coolant Temperature (ECT) gauge sending unit.
Drain the engine coolant until it is just below the level of the sensor into a sealable container.
Detach the gauge sending unit electrical connection.
If the sensor is held into the coolant passage with a retaining clip:
Remove the sensor retaining clip, and pull the sensor straight out of the housing.
If the sensor is threaded into the coolant passage:
Loosen the sensor using a crowfoot wrench, a suitable boxed end wrench or a thin-walled 3/8 inch drive 12 point deep well socket and breaker bar.
To install:

If the sensor is held into the coolant passage with a retaining clip:
Install a new O-ring on the sensor.
Using a small amount of fresh coolant or an anti-seizing compound, lightly lubricate the O-ring, and push the sensor in the housing.
While pushing inward on the sensor, install the retaining clip.
If the sensor is threaded into the coolant passage:
Install the sensor with a new crush washer. Tighten the sensor to 12.5-14 ft. lbs. (17-19 Nm).
Plug in the harness connector to the sensor.
Refill the cooling system with the proper type and mixture of coolant and bleed as necessary. For additional information, please refer to the following topic(s): General Information and Maintenance, Fluids and Lubricants.
Reconnect the negative battery cable and re-code the radio as necessary.
Start the engine, allow it to reach operating temperature and check for leaks.

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I'm sure you will need to move a few components to gain access, I am not certain which ones on your specific vehicle. It is on the rear of the right cylinder head. I'll look for a better diagram for you.