Where is the thermostat on a 1999 Audi a4 1.8t?

Normally under a bolt on cover where the top hose meets the cylinder head or built into a housing along the top hose.

Follow the upper rad hose from the rad to the engine and it is in a housing there. When you replace it make sure the spring goes toward the engine.

Depending on make and model, it could be under the housing where either the upper or lower radiator attaches to the engine. Example: Dodge 2.7L, 3.7L, and 4.7L it is under the lower.