get water in trunk every time it rains. I think it gets in through the tail light cause the side has a dent.
There is a cooling line that runs from the lower part of the radiator to the back of the car and up through the trunk. Maybe it is damaged and leaking? There is a shut-off valve right where it pops into the trunk (unless you have a Saab or most GM) so it’s easy to check. I think it’s meant to heat the trunk.
I’d go over it with a water hose to make sure where its coming in. Check the trunk gasket and look for any grommets in holes that may be out. Otherwise if its the tail light, go down to Napa or similar and get some strip caulk or non hardening body caulk. Take the tail light out and make a seal with the strip caulk. If you aren’t going to fix the dent, at least seal it up.
There’s a cooling line from the radiator to the trunk? I guess that is possible, but neither of my cars has any cooling lines from the radiator that go to the trunk. OP: What is the make/model/year?
I expect the OP is correct, this is rain water getting in somehow, maybe through that dent. Mabye get the dent and tail light fixed? Or at least tape it off nice and water-tight, see if that temporarily fixes the problem. At least you’d know what to do to fix it then.
Fatrap- I wish there was a flag for nonsensical answers.