I don't think your folding roof rack will provide a good ground plane. Others have mounted them on the spare tire mounting frame. Still others have used a mount off the front fender. If you haven't checked out K0BG's mobile page (
http://www.k0bg.com/) I suggest you do so. The Atas 120 is not a real efficient antenna so it needs the best ground it can get. It also has some weatherproofing issues you might want to deal with before you mount it. Google Atas 120 and you'll find discussions of that