Help ! Water Leak!

I’ve had my 2009 Peugeot 107 for over a year now and since last autumn I’ve had a leak in the passenger footwell. Loads of rainwater keeps flooding it and it’s constantly damp (since it’s constantly raining in England fml). We used some dye and towels and thought we’d found that the leak was a result of a loose seal at the bottom of the door frame, but once being sealed it’s still letting water in. My friend suspects it’s coming in through the back left light, but I don’t see how this is possible. Has anyone else had this issue?