Water leaks

Hey there. I have a '70 fastback. . over the past few wet months I have been discovering water leaks. I managed to find the cause of the bad leak (notorious sloshes of water 1 litre at a time onto the wifes lap) - being blocked drain pipes. I've recarpeted and all seems fine.

Although, I have noticed on the passenger side heater vent - under door sill into cabin, seems to have water sitting in it whereas the drivers is dry. Leads me to believe there is a blockage there too. Since that is a sealed unit where can I trace the blockage to be? I'm guessing there is a exit point somewhere.

Look at the left bottom corner of the screen. If it's leaking there it runs into the demister vent and down to the floor vent.

DH

It does drip slightly in that corner with heavy rain - thanks