To find the holes for my screws I went under the car, found the mounting points, then rammed a philips head screw driver through. Then I made the hole bigger so that the bolt would fit. As for the outside ones, they are a couple inches behind the seat riser. You should be able to feel the hole through the carpet, as I was able to.
1966 Mustang Sprint 200 "Oliver"... factory fog lamps, factory rally pac, off the road for restoration.
2005 Mustang 4.0 litre... Airaid CAI, Honeycomb taillamp panel, more mods to follow.