rattles and vibrations

I am wondering what are all of the options to help eliminate unwanted vehicle vibrations and rattles. Everyone has heard of dynamat and other similar products. But I have an 04 BMW that the speakers are mounted directly to the door panels and rear deck panels instead of  the stiffer sheetmetal as I am sure many other vehicles are. The problem is that the speakers themselves vibrate the panels due to insufficent mounting and thin panels. I have purchased some dynamat but am wondering if that is the best thing to use on the back of my panels. (molded wood pulp and plastic)(Is it enough?) I have thought about fiberglassing the backs of the panels to stiffen them up but that could turn into a real mess especially if it doesn't work. Also someone suggested to me to use foam between the panels and the sheetmetal to dampen and stiffen them. I have also seen expanding spray foam mentioned.

I would really like to hear from everyone who has dealt successfully with this all to common problem so that I don't make an unrepairable mistake.

Thanks in advance

dynamat and other products like it are great for stopping metal vibration, but it will not always get rid of it all. It is a great place to start, but in my old celica I had to make the area between the outer quarter panels and the frame a solid mass of spray foam. I also did dynamat on every bit of metal in the trunk that I could reach, and put a piece of foam weather stripping between every plastic panel. the only rattle that still was there was the rear window wiper.