My 1996 4 door Jimmy leaks windshield wiper fluid only when the liftgate is down (opened) or when the car is on a slope (rear pointed down). Any suggestion what to fix?

I had this annoying problem for a few years now, but I think it happened after I had the dealer replace the rear wiper motor four years ago. I can't say for sure how soon after that I started to notice the problem or even if the problem is related to replacing the wiper motor.

Anyway it would really be nice to solve this problem so that I don't dread opening the liftgate or parking on a slope. The leak will drain a full tank of windshield fluid.