If you want to switch the exterior light on independant of the interior light, you cannot take the feed from the existing light switches. You must get the feed either from the fusebox (if you have a seperate one inside the garage), or you can quite safely 'tee' off the feed cable to the interior light. You can tee off anywhere convenient along the cable between the fusebox(if you have one) or its entry point into the garage, and the first fitting (switch or light)that this cable comes to. Best way to do this is buy a wall mounting box with a plain cover. Switch off the electric. Cut the cable where you have positioned the mounting box. Put the cable into the box using the entry holes. Use a terminal block (a lego block with screws)to splice the new cable onto the existing (this will be coming through another entry hole) and fit the lid (for safety and it looks much neater). Then just wire up your light and switch. Of course you should wire up the light and switch first, then splice last. That way you can still have the garage lights on and see what you are doing!!