2001 Prism trunk water leak

I have no idea where it's coming from. This is usually one of those things where you have to look to find it. I'm afraid to tell you to crawl in the trunk with a flashlight while a friend pours water over it because I don't know the caliber of your friends. However, if it were mine I would drill a hole in the bottom area so water could drain as standing water will promote rust in the entire trunk area over time. These problems can be chronic and at least you will reduce damage this way. Make sure you don't drill into something critical like the gas tank. I would suspect trunk alignment is off. You may be able to adjust the trunk so it closes tighter. You could also test the tightness of the gasket by placing a thin piece of paper in the sealing area around the perimeter and be sure that the seal is pressing on it. It should feel a little snug when you pull on the paper.

I agree with strips of paper to find non sealing gasket.

Chalk powder or charcoal powder is also a good detective, pour on gasket, close lid, open and look for pattern.

Another way is to put a high powered cordless light in trunk, do this at night, in a dark place, the leaks will glow !

Rear window is a possible culprit, soak outer seams with soapy water, use an air hose from inside, poor seal = foam

Thanks for the advice, guys.