There is another thread on this topic. It shows how some owners have been able to push the fender liner inward enough to access the fastener with a socket. I was not able to do that. What I did is similar to what the Viper Team has done to their cars. After removing the fender I cut a notch in the fender liner to match the cutout in the fender. I then added a small black piece of plastic to cover the hole and prevent debris from getting in the gill area.
Note that to do this right you need to have the fender off first. That will be painful. You need a small open end wrench and need to flip it after each small turn. It is an absolute ******. One you have the opening made it is easier to change the sills than a Gen I/II.