One problem the first generation Colorados had and mine had was a leak in the third brake light. This would allow water to come down the back wall of the cab and soak your rug behind the drivers seat and move forward. I'm not sure it sounds like your problem, but it was known to be a problem with the third brake light seal. There were also leaks in the earlier models through the fire wall into the front drivers floor. I would ask in the first generation section and maybe a moderator could move your post. Your truck is pretty new to have the rocker panels rusting though. My 2007 was was 9 years old without any visible rust.
2016 Chevy Colorado Z71 V6 4x4 Extended Cab
A.R.E. Z Series Cap
Last edited by JBT; 03-21-2017 at 12:32 AM.