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it can be a few things:
1) and most likely - the gasket between your taillights and the metal are leaking water into the hatch. Remedy: Replace the gasket

2) There are drainage pipes that goes down the sides of the hatch (behind the panelling). Once in a while those can get clogged. Remedy: Inspect the piping and unclog if you can or replace.

3) The weatherstripping around the hatch opening is old and leaking. Remedy: Replace the stripping with new stuff or try reconditioning it to bring it back to life- that might work.