If you're installing an ordinary power amp in the trunk of your 'stang, then do your best to find a nice flat piece of metal that doesn't have 6 spot welds between it and the body. Scrape off the paint and VERY securely fasten your ground connection to this area with a silver or gold screw -- do not use black or black oxide screws. [It goes without saying to check under your car before drilling holes in it!] After tightening your connection, attempt to wiggle it -- it should be wiggle free.