I have some Panasonic 4" in front, they aren't exactly flush mount but they
just clear the window crank.  They are actually part of a component set but
I don't use the tweeters cause with a cassette deck the super high
frequencies do not matter much.  I also have these Panasonic three-way
satellite type speakers that I mounted to the side corners in back, I had to
drill a hole through the body into the air intake on the sides.  I also have
a set of Kenwood 6x9's under the back seats sort of as subs.  I have a Sony
tape deck and I have a Usacoustics amp running the 6x9's and the front door
speakers and a Kenwood amp running the three-way speakers in back.  I had to
use the Kenwood for those cause the Usacoustics amp did not like the common
ground connection that VW has running up to the front for the back speakers.
I have the amps mounted under the drivers seat and you cannot see them at
all.  I just got this all up and running last weekend and it sounds great so
far, just need to do some tweaking with the levels and stuff now.  You can
find speakers to fit the spots in front but it might be harder to roll up
and down the window.  I guess you can fit up to 5 1/2" speakers in those
spots.   Tim Vogelin