on the interior side of the hatch, directly under the spoiler there should be a long piece of plastic held on by like 4 screws and 4 of those plastic push tab thingys (cant see the push tab thingys till u take the screws out and pull at it).
anyways, im not sure if this is the piece you are talking about but when had a saleen spoiler water somehow made its way under the spoiler and would collect on the plastic trim piece. When i opened the hatch it would all rush out that defroster hole (the trim piece partly covers the defroster wire if i recall correctly).
I dont think the plastic trim piece covers any part of the glass though...but i dont have it on (it rattles-im missing screws so i took it off).