Star by flipping up the carpets on all sides to see which is wet or damp.
If it is from front passenger side, check if the A/C evaporator drain hose is plugged or detached. If you have coolant losses, then it could be due to leaks in the heater core, leaks might get to driver sid as well.
Next to check would be door trims cover. Behind the door panels are plastic weather proof seal which would allow water to get into the cabin if they are not properly sealed.
If any water accumulatingpoints are encountered, with carpets flipped up, use a garden hose to test for leaks, starting at the lowest point of the area near it. Visually inspect for leaks and repair accordingly.