I'm always scared to poke/cut a hole through the grommet. Knowing my luck, I'd cut through several wires as well. I actually tried this route -- by poking an awl through. The rubber was very tough. I would have taken a sharp knife. I wimped out, and went through the fender, and factory grommet by the kick panel (easy, and no one sees it anyway).
You can barely see it in this pic:
(missing photo)