I had the same problem with my 99 ob wagon. I found it was coming from the heater motor (pasenger side) water was draining from the spongie filter thingy (sorry for the technical language, but you'll see it if you poke around). I tried taking it apart but with little luck. Finally broke down and took it to the dealer who couldn't find anything after a leak test. I figure it was leaves or needles that got jammed in there via a drain because sometimes I could hear them when I turned on the fan and somtimes little bits would shoot out the vents. Anyways the problem eventually fixed itself and has never returned. I try to keep all the drain ports clear of leaves (using a hose, you'd be surprised with the amount of crap that comes out). Sorry I couldn't be more help but other people have had the same problem.