Could be the water shield in the driver door, or a leaking sunroof drain tube. To see if it is the drain tube, slide the glass rearward and look for a drain hole in the front left (driver side is always left) corner of the sunroof frame. Pour water down that drain hole. If it comes out in the wheel well just behind the front left tire the problem is not the drain tube. If no water comes out, the drain tube becomes the prime suspect.
If is not the drain tube, remove the driver door trim panel and examine the water shield (large sheet of translucent plastic) to make sure it is properly adhered to the door sheet metal, and not pulling away anywhere. It is stuck on with butyl tape (nasty, sticky, black stuff) which can dry out over time, causing the water shield to pull away. If that is the problem, go to a store that supplies stuff for doing body work and ask for a roll of the black sticky stuff older cars use to install the windshield. Get the smallest diameter product they have. Remove the water shield, but pay very close attention to how the old butyl was routed. There are drain holes on the bottom of the door that need to be within the seal, but not blocked. Remove all the old butyl (this is where you will discover why I call it nasty) and re-install with the new butyl. Before you put the inner trim panel back on, spray the outside of the door with a hose and abserve where the water goes.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)