Dynamat or an alternative will help you get rid of the trunk panels vibrating. A couple layers on the underside of the trunk lid, along the trunk floor, and the side and rear panels should do the trick. First, find out first where the rattle is coming from, and work from there, you might need more layers on a certain area than another. Also make sure the spoiler, taillights, and licence plate are tight. Dynamat extreme is an excellent product, but can get expensive. Some alternatives I'd recommend are Brownbread, B-quiet, Raammat, FatMat, Secondskin, and eDead. Most are about $1 per sq. ft.