We prefer seat belt mounting bolts also but I agree-we have been bit for one reason or another by a bad ground at these points. Never can find a good rhyme or reason for this. Then we use self tappers and lock washers also as a backup if the seatbelt stuff doesn't work.

Problem with self tappers is they sometimes come loose or strip. We have had the best luck with actual bolt headed self tapping sheet metal screws and lockwashers from Home Depot. I have yet to have one of those strip out or come loose.

On really big systems where big bucks are paid on the labor side of the install, we still tack the terminal to the car's sheet metal with the welder after it is bolted in (especially if the ground is located somewhere that will not be easily accessible after the build-ie: under fake floor, fiberglass enclosure, spare tire well, etc).