Well, none of mine are. There is a proper rounded profile shaped rubber gasket held in a groove moulded into the inner face. Fixing is solely by 4 plastic clips which are part of the plastic moulding. Often people yank them out from the boot-side without releasing the four clips ('cos its a bumper off job) and then glue them back or fix them with foam rubber and pop-rivets or some-such. Thereafter they leak forever. I'm pretty sure they are the same on all 75's / ZT's?
Water gets to them by running down the outer wing panel to the join with the rear bumper "arms". That join is no way water-tight, and is not intended to be. In fact the shape of the lower wing body-panel and that of the bumper inner surface almost directs the water then onto the vents. If you use a hose on the upper part of the wing you can see exactly how and where it gets through!
David