Thanks. I'm not sure how a C5 door sill is held on but you are correct on the C6 - Metal tabs (clips) into the fiberglass floor. In playing with it we saw "strain" on the plastic as we just tried to pry it. Once we pryed it a little and got to see under and saw the design, we saw where we could insert a flat blade into the metal clip and release some pressure and raise the plastic sill.
The project was a group of us installing turn signal mirrors on the C5s and C6s. One of our club members had done his C5 and was "guiding" the others in the project. The C5 process did not require us to get under the door sill. However, the C6 has some wires up near the dash at the sill level that had this requirement. Thus, the question.
Thanks again. Hal