If you think the drain is clogged, I would first remove 1/4 panel trim and search for sunroof tube. I believe it runs down pillar behind rear door. Then blow compressed air in the lines. First, Open sunroof fully. Then take a coffee can full of water. Pour water in track assembly, it should leak out immediately. If not, drain is clogged. No need to remove headliner. I do this for a living in New Jersey. Havn't done a wagon in a while though. You must find end of drain of course.