has it got any screws coming out of it or any holes in the end for bolts to go in?
if not then your gonna need some Sikaflex, basically a high bonding substance to use in a mastik gun. put a big blob on each corner and perhaps one in the middle and masking tape it to the roof. make sure you push it down firmly and that you got no excess pushed out.
if it has got bolts your gonna need to either pull the bolts out n smooth it off, or drill through your tailgate after knowing exactly where it ligns up.
completely upto you which way you do it