Ok I have an 04 Monte Carlo SS Supercharged that is making me nuts. I have this bad leak in the passenger floorboard that I can't figure it out. I thought it was the heater core but the heat works really good and the leak that I am having isn't coolant it is just water. This doesn't happen when it rains. This happens when driving the car. I tried unclogging the a/c drain plug but didn't help. Like I said this issue isn't from the rain it is when car is driving or running. It seems like the leak is starting from the middle console under the a/c control unit.

 

I have ran into a similar situation 2 times in the past both were condensation in the heater box inside the car. It happened when someone prior to me putting in a heater core and.breaking the box installing. What happened was the water wouldn't make it to the drain at the bottom.If that's not the issue the only other thing it could be is water entering by the cabin air filter because the seal is not on properly. I understand that you said it's not rainwater but there's only so many possibilities. Good luck Scott