In my '93 DeVille, I drilled a hole from the underside right up through the Package Tray and conveniently into the small storage compartment there. I then used an Eye Bolt with Nuts and Large Washers on both the top and bottom sides. The bottom side got a Double Nut for some extra security. Not sure what car you are working on, but if the metal under the Package Tray is strong enough, it should be fine. I got lucky as my tether is hidden when not in use. The only difference is that the car seat is in the middle of the rear seat, instead of to one side.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America