6-cylinder engines:

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On cylinder block at vibration damper side -arrow-.

On the 6-cylinder engine the number is visible by looking through from the front right between suspension strut tower and air cleaner.

Fig 1: Identifying Engine Code Letters And Engine Number Location (2.8L - 6-Cylinder Engines)

Courtesy of VOLKSWAGEN OF AMERICA, INC.

Notes for all engines:

The engine code letters can also be found on the vehicle data plate, located in the rear left side of the engine compartment.
The engine code letters and engine number can also be found on a sticker on the camshaft drive belt guard (4 -cyl. engines) or on the left cylinder head cover (6 -cyl. engine).