4000 Quattro Water Leak in Trunk

[Dave Fain <KC3565L@sprintmail.com>]

I just joined the list so pardon me if this is one
of those questions that has been answered numerous times.
My recently purchased 1985 4000 Quattro collects water in
the spare tire well.  I removed the rubber plug and drained
it, only to find that it had about 2" of water the next time
it rained.  The car has never been hit and the rubber seal
seems to be in good condition.  This time I left the plug
out and made a small hole in the mat so it will drain, but
I would like to find the source of the leak.  Is this a common
problem with an easy solution?
Thanks in advance,
Dave