Last year I bought an used 2007 Equinox a few months later I noticed the carpet in the back seat was wet, I assumed my husbands loser friend spilled something and didn’t say anything about it. Later on I heard sloshing noises when I turned the corner. I opened up the spare tire storage area and it was full of water. I found out the dealer had replaced the back window because of a recall, so they replaced it again thinking that was it, but it wasn’t, I still get water in there every time I go through the car wash and I’ve never seen the windows leak, obviously it isn’t coming from underneath, or it was just leak out, my husband thinks he is just going to drill a hole in the spare tire compartment. I just want to know where it’s coming from and fix that instead.
You have to assume that the rear window is leaking. You might not see the water running down the interior of the rear window, but it almost certainly the cause. If the car was older I would suspect rotted floor boards, but it is too new to look at anything except the work that has been performed. In lieu of drilling holes in the spare tire compartment, see if your husband can access the metal directly under the rear window, and then hose the rear window down for a few minutes with a garden hose and see if metal under the window is wet.
The drilling a hole idea sounds good but your gas tank is right there. DO NOT drill from the trunk down.
Do you have a sun roof? If so I would give it an 80% chance of being the source of your problem. They have four drains, one at each corner. If one get’s blocked it will leak.
String trimmer line and compressed air, gently used is the recommended fix. GENTLY as you can damage it.
Sun roofs are not designed to be water tight, they are designed to allow some water to get past the seals and to capture it and drain it under the car. There are some more complex problems like drain hoses coming loose etc. as well.