Not difficult at all, but you do have to remove the rear seat bottom and seatback. The Haynes book will step you through it. No special tools are needed for the rears. For the front ones you will need a 10mm 12 point socket.
2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg
And remember folks, Only You Can Prevent Hilliary.