My 13 year old cap was somehow unable to keep a good seal on its place on the top of the plastic coolant tank in the rear trunk.I noticed a little moisture around the base; then I noticed "sweating" on inside the trunk area directly above the cap area.
My coolant level had been very slowly dropping.I read about all of this on this forum
and found that an improved cap is available from Porsche.Look at the part number on your cap and search this forum for the number on the updated cap.I'm thinking the last numbers are 00 and my new cap has last numbers of 04.Research it and ask your dealer if they have the updated cap and the part number.I ordered the new cap form a Porches dealer.Maybe 20 bucks or so.
No more leaking or moisture on the inside bottom of the trunk lid.When you replace the cap,make sure your are not cross threading it or over tightening it.Good and snug is enough.When you get your cap you will see that it has a longer tube shaped base.