I had the exact same problem on the same side. I removed the plastic cover, removed the old sealant, and put new mastic sealer in there. But to no avail, it kept filling up regardless. A good way to find the leak is to use a spray bottle full of water.
Lightly mist the problem area with water, INSIDE the car, then use something along the lines of talcum powder. Put the powder on a tray and use an airgun, or even your breath to blow a fine layer of powder everywhere around the offending area. Let it dry, then hose the shit out of the OUTSIDE of your car and you'll very quickly find the EXACT route the water is finding its way in, as there will be a clear run through the evenly spread powder. Helped for me anyway.
Good luck