were to drill a hole in boot for wire from towbar?

Assuming there are no rubber grometts in the area you can utilise, there are some important rules.
Do not drill any holes inside or immediatly behind the wheel arches.
Do not try and drill through from the inside of the boot without knowing exactly where your drill is going to end up.
Do not drill within 12mm of a spot welded seam.
Drill a hole large enough not only for your cable but for a plastic grommet to protect the cable from damage in the drilled hole.
Your tow bar electric cable needs to be long enough to reach behind both rear lights, unless you know where the wiring loom is located, because the Black wire will be the feed from the left side light while the brown is the feed from the right sidelight (dont join the black and brown together)
For the full cable colour code-Google 'wiring digram for 12N socket'

GB in Wales
March 2013