Trunks are complicated and become more susceptible to rattles the older they get. You really need to study the problem - vibrations dampers like Dynamat Xtreme will lessen the energy available to a rattle, but there are two hard pats hitting each other and you really need to find the sources and treat them before you even bother with sound deadening.
License plates and spoilers are classic sources. So are taillights and locking mechanisms. You may improve things by adjusting the bumpers or latch tightness. Replacing worn weather stripping might also help.
Once you have tightened, taped and padded the problems, add sound deadener and you should be golden. The structural modifications suggested above might be worth exploring too.