If you purchased your Sprinter with the insulation option, there is allegedly a kit - 1 1/2 inches of light gray open-cell stuff glued on and very skimpy (some people report that Sprinter's insulation kit is OK - maybe in later years, mine is a 2002). But mine has it, and it is useless. The side door panel comes off easily (take the handle off, then carefully pop the plugs out). Then, start with a dense sound deadener similar to Dynmat (find something cheaper) and hit the door's impact areas. Then go after the empty space with your choice of bulky insulation. Pull out the crash barrier on the bottom of the door and deaden around that area and replace. But be careful not to get insulation in the very bottom of the door - the weep holes have to stay open. If you have your stuff at hand, should take no more than 20 minutes - and you don't need to take the door off (seats out of the way is nice).
