Right-angle drill & short bit is one way, as mentioned by Squareparts2. You can also use a spring-punch from the inside. It will put a very small divot in the metal, from the inside, that can be picked up on the outside. After finding the first hole, I use a Unibit from the outside to find the first hole. Carefully drilling out the hole with a Unibit, will find the center of the first hole, relative to the backing plate. The rubber mounting pad, serves as a good template for the other two holes.
Backing up these bolts can be a bit daunting, so using a Riv-nut is also a good solution, in lieu of a sheet metal screw or bolt & nut. Riv-nuts and the setting tool are available at most well-stocked hardware stores. I don't recommend using sheet metal screws, since they are prone to pull out.
