>> Any suggestions for 4" replacement speakers?  How is the factory
Wolfsburg-Edition "subwoofer" wired in; is there any sort crossover
arrangement? <<

I found some Polk Audio's that were a direct replacement for the door
speakers. They sound pretty good for the size and were probably < $100.  I
did need the handle crank extenders to get the handles to completely clear
the speakers grill though.  I did not mind a little "thump" when I was
opening or closing the window but my wife gave me "the look" when she tried
to open the window.  Worth the exorbitant price of $29 a set.

Bryan