Happens to me too. It happens on a lot of GM cars, I have been told. My friend'* 97 Buick is worse than my Bonny. I don't know why it is, just is I guess. Maybe someone else has actually looked at it more than I have.
Will try to look a little more into it myself. Is it a real soaker when you open the trunk? If you hold the lid ajar for a moment, does that help?