Engine number, location
WARNING: This page is about a different car, the 1999 Volkswagen Jetta, 1998 Volkswagen Jetta, 1997 Volkswagen Jetta, 1996 Volkswagen Passat, and 1996 Volkswagen Jetta. However, it is still accessible from the selected car via links, so may be relevant.
The engine numbers ("engine code" and "serial number") are located on the front of the cylinder block between the Diesel injection pump and the exhaust (arrow).
There is also a sticker on the toothed belt guard showing the "engine code" and "serial number".
The engine code is also included on the vehicle data plate.