You can check the taillight lens seal - they can leak.  But I had
a persistent problem on my (non-cab) E30 which also leaked
into the trunk.  It turned out to be a broken tack-weld!  Look
in the channel where the trunk weatherstrip goes.  Along where
the rear window sits (with top up) you should see a tack weld
on each side.  They are outside the weatherstrip.  My driver
side weld had broken loose creating a small hole for water to
enter.

Bob