Remove all your trim back there and look for water trails. It sholdnt be too hard to find with the wonderful Mopar paint of the time.
By the time it's 15 years old, it's time for a new weather strip back there anyway. Most competent glass shops (i. e. not in my local area) should be able to get it for you. There is also an ebay vendor called the weathertripstore, or something like that, that I got my door weather strips from. Pretty good quality and prices. May not be OEM quality (I dont have new OEM to compare to) but a world better than 15year old OEM.
Daniel