[projectRDM]

If the window has never been replaced (that's OE moulding so I doubt it) it's not leaking from there. 99% of trunk leakage is from the tailights, the butyl rubber sealant dries out and becomes brittle, allowing for water to leak through, especially if the tails have ever been removed. You can also have corrosion around the trunk weatherstripping, if the metal rots enough it will leak underneath the strip. Check both of those first.

To answer your question though, the dam is a rubber retainer that the moulding snaps into which is glued in with the window. The moulding can be easily replaced as long as the dam is still good, otherwise you're better off gluing the new moulding in as it would require pulling the glass to properly replace the dam.