This is a common problem as you already know.
When you open the trunk after a rain or wash, look at the
sockets for the license plate lamps. You will probably see
the water coming from them. You have to remove the bracket,
lens/reflector assembly from the trunk lid and reseal it
with silicon. You could try and replace the gasket, but its
probably going to start leaking again. While you are at it,
remove the taillight lenses and reseal them.
I have seen lots of posts on this over the years and I know
that other places do leak. These are the places that I had
to fix on my 98 Mys.
jeff
