Volkswagen: 1984 Jetta Dash Leak, plenum chamber, water runoff

Question
Hi John.
I am the proud owner of an almost-perfect 1984 Jetta.  The only problem that I seem to be having is that every time that it rains (or I wash the car), water leaks from somewhere under the passenger-side dash, next to the air conditioner unit.  There is a little vent down there that seems to be the output for the leak, but I don't know enough about cars to find the place where the water comes in.  It has only been doing this for the last year or so.  I am wondering if there is a water runoff tube or something in the vent that is clogged with leaves.  When I run the fan unit (for the vent or heat/AC etc), it is obvious that there are a lot of leaves and twigs in there.  If you could instruct me how to clean that out also, that would be great.  Thanks a lot for your help.

Answer
a common problem is the plenum chamber (area beneath windscreen) has drain holes and tubes which become blocked. this then allows the plenum chamber to fill up with water, which overflows through the heater box into the car. the only other possibility is a perished or loose grommit, or worst case is a rusty hole in the plenum chamber or bulkhead. you will need to remove the covers under the windscreen (under bonnet area). then you may need to remove the battery to find and clear all the debris away. this may be better attempted by a mechanic if you are not mechanically minded.