2005 Chevy Monte Carlo leaks when it rains on passenger sid

Check your a/c drain on the heater. You need to reach it from the bottom. It will have a hose runing down to the outside. It may be partially plugged and then draining onto the floor when it gets alot of water. You'll need a coat hanger type thing to stick up fromt he bottom and try to unplug it. You may get wet doing this. You'll see it come through the floor to the outside.