[Stockholm 300C]

My neighbor has a Hyundai with the same problem.
This is his way to find the leak. He is in the trunk with a spot light. I am outside with the hose. I'm going to town on the car for a while, then stop. He shines the light and looks for water. It took a long time, but the leak was finally found. It war coming in through one tail light opening (?). Not much, but enough to cause damage over time.

Give it a try. With a spot light any water dripping will shine like diamonds and will be easy to spot. But be prepared to spend a good half hour or more in your trunk (LOL).