If you do have condensation inside the tail lights then some water definitely is getting in somehow to the tail lights. It doesn’t necessarily have to make it into the trunk though.

For me, on a regular Corolla and not a hatchback, after much hunting and spraying water everywhere, I solved my water leak by replacing the weatherstrip around the trunk lid.

 

Only possible location would be through the hatch shell. I’d remove the trim panels and see if it’s running along the seam sealer for the hatch. Might even be at the hatch strut when it’s closed.

 

Could the bumper mounting holes be the cause? I put some silicone there last time but might have missed the upper hole. I think condensation in the tail light is a separate issue and might not be the cause of water in the trunk. It's an old car so I don't think it's worth using much money on this repair. I'll need to remove the interior side pandels and check further. It's so difficult to pin point the leak.

 

I removed the side panels. Here's where the water is collecting. The stain in the middle is something old.

Looks like some water gets in from here. But I don't know where it came from.

I also found water on both side. I think it's been like that before as well.
No water trail from tail lights, bumper mounting holes

Could seams under weather strip on the roof cause the issue?

 

Inspect your weatherstrip for damage or binding. The gutter in the weatherstrip will sometimes collapse and won’t make a water tight seal.

When I used to work on the assembly line, some didn’t sit flush with the body. The channel or gutter would adhere to itself.

 

I found the spot. Not sure how the water gets in there though. A seam behind the tail light? Or maybe from the roof?

There's a light rain today. I noticed the whole rear window is wet. Any advice how to tackle this issue? Is it just condensation or a leak somewhere? I don't think there's a lot of water present in the car for such an amount of condensation. Maybe worn window seal where I marked blue?

 

I can’t tell if that is a channel you circled in blue. It looks like a channel to me where the bead of water can flow into. You should use a fog machine inside the passenger cabin to locate any spots where moisture and water can enter.

For the window, the seal around the window might be brittle or cracking to allow that much moisture and condensation in.