After growing tired of airing out the perpetually soggy trunk mat in my 1978 BMW 320i, I traced the water leak to poor design of the sealing of the tail light housing to the body.
The lateral aspect of this housing fails to seal in the area adjacent to the side marker light.
Removal of the tail light housing, cleaning of the area, and application of a judicious amount of window putty in the problematic area sealed it up as tightly as a U-boat's hatch.
Chewing gum should also work fine and be marginally tastier.
-- Eric J. Dierks