Do tCs have a rubber plug that could be removed so you can access it to remove dings and dents on the rear quarter panel?

no.. but you can remove the entire interior panel.

In the back seat, the panel there you mean? or the door panel? How hard is it to remove? The door panel won't help me. There is def not a factory hole that was silicone plugged to access?

Just take off the rear panel next to the back seats and it will give you full access to the 1/4 panel.

Here is a tech on how to take apart the interior

http://www.scionlife.com/forums/viewtopic.php?t=60981

you should be able to gut the interior in under 90 minutes even if yo never did it with the directions listed above.