Looks like you and I have the same problem, but posted in a different area. Luckily for me Steve was nice enough to respond to me as well. My thread is here.
It has some descriptions of the part numbers and might be a good link if someone posts to one or the other.
I replaced my rear sealing strips (that is the GM name for the window sill pieces) when I replaced my regulators in the back. It made taking the sealing strips out easily because the window was removed. If you do remove the window, it would be a good time to fix/prevent failure of the regulators by following the links on this page.
When I removed the strips it was pretty much a brute force thing. Try to find a weak spot where it will pull up, then work that to get it all free. You do have to be careful not to bend the piece as it will bend if you put too much force on it.
Good luck, I look forward to seeing how you do with the repair. I plan on giving it a shot when the weather here starts being less wet!