It has also been a common problem on the M's on the front doors, though
my '99 Concorde (same doors - except some of the trim comes molded to
match interior color - perhaps that's the difference?) has never had the
problem.

Besides DI.net that Greg mentions, you might check out the 'Black Trim
and Wetherstripping' forum on the 300M Enthusiasts Club forums
(http://300mclub.100megs42.com/forums/index.php).  Pretty much that
whole forum has been on the subject, though not many recent threads
because the subject's been pretty much beat to death.  Because the
weatherstripping is so expensive (if, say, you need to do all four
doors) the solution that seems popular is to buy one door's worth of
weatherstripping, remove the  threshold, slide the ends of the
weatherstrip apart, fill in the gap with a piece cut from the new piece,
and re-install the threshold.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
adddress with the letter 'x')

Yes - the door gasket on the M's and other LH's has a tendendency to
shrink and pull away from the frame leaving air gaps.  Read my earlier
reply for pointers to further reading, and a popular (money-saving but
effective) solution for the problem.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
adddress with the letter 'x')

(top-posting fixed)

I've got the same problem with my wife's 2000 Intrepid R/T.  I "sorta" put it
back up (like you say, it's like it's too short) every few weeks.  Not
a serious problem, but annoying.