The links below are for the combined WIS to do a 3rd row seat retrofit on any model year ML and a publication written by "cgeorge430" on benzworld.org.  The publication was written for MY '98-'99 MLs but is virtually the same except for the necessary modification to the 40% seat to allow for 3rd row entry on newer models.  That information is included in the WIS.  Pay close attention to the VIN callouts found on some of the WIS pages because not all instructions are required or needed for all MLs.  The 3rd Row Seat kits are no longer available from Mercedes-Benz dealerships in the US for MY 2000 and above AFAIK.  I bought mine from Engisoft late 2003 for under $1700, although they have since raised the price on the kits because they are so hard to find.  I have not been able to find them anywhere else in the US.  If you find a place let me know and I will post it here.