Re: motion sensor wiring
If you go to the side of the door, you shouldn't have to remove any drywall at all, just cut your new light fixture hole for a 'cut in' style box and pull the switches out of the interior box, leaving them attached to their wiring.
This should give you enough space to snake a new wire from the fixture to the switch box, you may need a helper, and it might be easier to back fish a smaller wire from the switch box to the fixture opening.
What is being controlled by the existing switches, and how many wires are attached to the switches and bundled in the box?
ALWAYS turn off the electricity before opening or working on circuitry.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!