Check out the stereo link on this page:

http://my.tbaytel.net/guskers/vanagon.html

I mounted speakers in the lower door panels.  The stock location houses a
high range speaker, the lower door, a mid bass driver.  The passive
crossover is mounted in a bad spot and gets booted every now and then.  The
mid bass driver is fine where it is.  The map pocket needed a slight (but
invisible) trim.  I had intended to fab up a custom panel that would house
the crossover and never got to it.

If you're sending some power to the door speakers, take some time and
dynomat (or equivalent) the door sheet metal.  I find that when the system
(700 watts or so) is cranked, the door flaps a bit.  The carat type bumper
extension trim on the lower doors is another source of vibration (makes a
racket outside the van), and is resolved by remounting with some
isolation.