a real cheap fix is called Ice Guard, or brands similar to it. I believe you can get it from Home Depot or a place like that. it's intened purpose is to insulate corners of a roof, but seems to work well for sound deadening applications
The ice guard stuff goes on just like any other deadener would. It has adhesive on one side that you stick to the car. Not sure if you have to heat up the Ice Guard, most sound deadeners require them to be heated so they get stickier before application. The first thing you should coat is the trunk lid, that was what rattled the most on my car. Next check your tail-lights, I actually had to take them off and completely fill them with foam to stop them from rattling.