[brcidd]

Completely possible it is from a/c vents-- water could be backing up and not flowing out the condensate drain-- the big hint to this is the answer to this question-- Do you drive for lengthy periods of time at highway speed- say 30 minutes or more, non stop at 60+ mph? If so, the next thing to look at is Do you have a right angle (elbow) tube on your condensate drain pipe? It would be nearly centered on the firewall, engine side in your car. If there is just a plastic pipe protruding out- with no rubber elbow pointing the flow down, then - there-in lies your problem. The rubber elbow creates a chimney effect, that sucks the water out of the evaporator drain pan inside the car- if no elbow, then wind pressure (from driving highway speeds) pushes the flow (or does not allow flow) out the horizontal drain pipe. Once the water backs up high enough, it flows out into the duct work- which in your case leads directly to the back seat vents.

I've seen this in GM B-bodies where the second shift assembly operators did not install this crucial elbow- salesmen who drove this station wagon complained of water on back seat floor only after a 3 hr non-stop highway drive- put the elbow on the drain (or fashion a drain hose) and the problem goes away.. Anyone who knows a/c can easily verify this issue.