It'* the rear drain tube from your sunroof.
It has become disconnected, and the water drains down the headliner, past the holy sh*t handle, then goes for a ride down the rear interior panel, by the back window, then ends up under the back seat. It then cascades down into the rear floor pan.
I went through the whole mess, check my post: Water water everywhere.
Now that I have re-connected my drain tubes with a small hose clamp, and am satisifed no more water is getting in, I cleaned the salt stains with carpet cleaner foam and a brush.
Drop your head liner, grab a flash light, and see if those tubes are connected!
2006 Pontiac G6, 2.4L Ecotec
1993 SSEi "Bonnesaurus" sold regretfully 10-27-10