on teh boot floor of mine there is a rubber grommet with a few wires sticking through, i drilled next to the wires through the rubber as it was quite big, then as tom says, over the fuel tank.
There is one in the bottom of the spare wheel well.
It is plugged with a plastic grommet. I drilled out the middle of the plastic grommet, pushed the wire through and then filled around it with silicone to stop chaffing (ouch!)