If your drains are like those in my '95 854, the bottom of the hoses end in a fitting that pinches the bottom of the hose.

If you park where there is a lot of debris from trees, the drain will clog regularly,

There is also a bend right after the hose begins at the top - careless use of something too stiff may push the hose off its fitting.

I have a piece of insulated wire (like automobile wiring) that I have marked for the proper length to clear the drain. I kink the end a bit to clear the first bend in the hose. I keep this wire in the trunk.

I check the drains with a cup of water every month or so, and they are usually clogged but do not cause issues unless it rains hard and I park facing down grade.

Also - check to make sure the trunk gasket is clean and the surface it mates to is clean also.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 485,000 miles put on 9 bricks