Where is the thermostat located on the engine of a 1994 volkswagen passat ( glx vr6 engine),I need to replace the thermostat BUT there are no repair manuals for this car.

Your car is a little unusual. Here are the exact procedures according to autozone.com. Let me know if you need the photos and I'll post them somewhere you can get at them.
All 4 Cylinder Models

The thermostat is located in the bottom of the water pump housing.

If equipped, remove the lower engine shield.
Place a large drain pan under the vehicle to collect the coolant.
Remove the radiator cap and drain the cooling system into a suitable container.
On 1.8L engines, remove the water pump housing drain plug.
Loosen one bolt at a time until coolant begins to flow.
Remove both bolts and remove the thermostat.

Fig. Remove the housing bolts

Fig. Once the bolts have been removed, pull the thermostat housing away from the bottom of the water pump unit






Fig. Note the position of the thermostat and gasket


Fig. The 1.8L engines have a water pump drain plug near the thermostat housing

Fig. An exploded view of the water pump, pump housing and thermostat-1.8L engine illustrated

Fig. The 1.8L water pump, drain and thermostat housing

To install:

Install the thermostat using new O-ring.
If removed, install the water pump drain plug with a new O-ring and tighten to 22 ft. lbs. (30 Nm).
Torque the housing the bolts to 88 inch lbs. (10 Nm).
Top off cooling system with the recommended fluid and concentration. For additional information, please refer to the following topic(s): General Information And Maintenance, Fluids And Lubricants. Your car is a little unusual. Here are the exact procedures according to autozone.com. Let me know if you need the photos and I'll post them somewhere you can get at them. All 4 Cylinder Models The thermostat is located in the bottom of the water pump housing. If equipped, remove the lower engine shield. Place a large drain pan under the vehicle to collect the coolant. Remove the radiator cap and drain the cooling system into a suitable container. On 1.8L engines, remove the water pump housing drain plug. Loosen one bolt at a time until coolant begins to flow. Remove both bolts and remove the thermostat. Fig. Remove the housing bolts Fig. Once the bolts have been removed, pull the thermostat housing away from the bottom of the water pump unit Fig. Note the position of the thermostat and gasket Fig. The 1.8L engines have a water pump drain plug near the thermostat housing Fig. An exploded view of the water pump, pump housing and thermostat-1.8L engine illustrated Fig. The 1.8L water pump, drain and thermostat housing To install: Install the thermostat using new O-ring. If removed, install the water pump drain plug with a new O-ring and tighten to 22 ft. lbs. (30 Nm). Torque the housing the bolts to 88 inch lbs. (10 Nm). Top off cooling system with the recommended fluid and concentration. For additional information, please refer to the following topic(s): General Information And Maintenance, Fluids And Lubricants.