We had some very heavy rain during the weekend that resulted in water ingress that accumulated at the driver’s side rear carpet area. The front, and passenger sides are dry.

Is there a drain plug, passage, seal, or other thing I should look at that could be the culprit of this specific issue?

 

Is your drivers window properly aligned? If the glass gets out of alignment in its channel or your weatherstripping becomes compromised in the wrong spot, that could be the culprit....

 

Along the same line of thought, I believe the bottom of the door has drains. If they are plugged, the water can leak inside.

 

Lift the back seat ...on mine I had about a gallon of water under the rear seat squab that had entered thro a split in the monocoque construction . dependent on the angle of the car it either goes under the seat or into the sill. When its full it overflows into the rear footwell area. If you drive and brake it slops over and runs under the carpet.
Could of course be door seal area.