3-4 cans expanding foam in the hatch lid. first you cover all the small holes with tape/cardboard, then take the license plate lights out and let them hang there by the wire. then cover the hole for the license plate lights the best you can with the wires hanging loose. spray the foam into the hatch door and let it sit to try for about 12-24hrs. remove all the cardboard and tape and put everything back together. adds weight and stiffens the whole back door. i did 2 cans on my last foci and it worked great. almost completely fixed it but i also had 2 12" t2 rockford subs on a t1500.1bdcp. cant really stop that rattle with that amount of bass. your single sub shouldn't take as much work.
hn155: Rockford punch pro speakers, t400.4 deh 80prs, full audio build to come. working towards 175hp n/a, simple but fun.