1986 4000S, Water on passenger area floor

[Peter Biernacki <pbiernac@ea.oac.uci.edu>]

The car in question is a manual 1986 4000S with a 1.8L 4 cylinder
engine (7/85 production date).

I'm afraid this is big trouble, but, the other day I discovered that the
footwell on the front passenger side was wet!  Pushing down on the carpet
revealed a puddle of water.  Can anyone help me with this?  I haven't
taken my Audi through the ocean, a la James Bond, in case someone is
wondering.  Is this a problem with the heater core?

Thanks,

Peter