Hi there i had the same problem on my drivers side with hardtop fitted, used to leak with heavy downpours.
Managed to fix it now though!
I also didnt have any leaks through the plug but took it out and re-sealed it , still leaked.
What i noticed though that when lots of water was put over the area the water would overflow the small lip that is present at the end of the a pillar and then run into the car (when i took the hard top off) This didnt happen on the passenger side as the lip seemed to force the overflowing water down the outside of the car over the small 1/4 front window.
My next step was to build up the lip on the drivers side slightly with a bit of silicone only by about 1/2 cm but hey presto when there was too much water for the two leak holes to manage, the excess water went down the outside of the car.
Hardtop back on and hey presto a dry seat.
May not be the same for your car but worked for me.