Stevega

I recently purchased an 01 S430. I am generally very pleased, but have had a problem with the trunk leaking.
The problem is on the left side, outside of the trunk opening. When I pour a cup of water in the channel to the left, the water leaks directly in. (The channel to the right works perfectly.)
I have not tried to remove the hard plastic parts to see the source of the leak. Is there a seal that has cracked underneath the plastic? Is there some gasket material there that needs replacing?
Thank you very much for your help!
Steve G