re:  ... we wouldn't need VW dealership parts counters except for a
few real dealer only parts.... I would think they know that...

Another way to say this is, "can you say franchise?"  On the other
hand Mercedes used to and may still publish entire books of parts
illustrations for consumer use.  They don't have part numbers, rather
reference numbers the dealer can match with the part number.  This
way, I can bring the book in and say "I want this... ," and it all
works more efficiently.