Well I have a 99ES as well and I was having the same problem. The Chrysler guy said it was the speaker so he replaced the speaker and tightened the grill and guess what no more vibrations. It's annoying when you can't jamm your favorite song.
Just popped it off, and bent a few of the tabs out to see if that would make it tighter. Seems to have worked, however, it is still not as tight as the driver door. But the vibrating seems to have stopped, for now
[ September 16, 2001: Message edited by: Intrepid99 ]