Welcome to NICO. One of the many great perks of being a member is access to the actual FSM for your vehicle.
In the DIY/FAQ section you will see a link to all the field service manuals. These give detailed diagrams and instructions for everything. I have posted the link to the section for 2008 dash board:
http://www.nicoclub.com/FSM/M/2008/IP.pdf
You will NOT need to remove all the parts shown. Just the parts of the dash required to allow you access to the NAV screen.
On my 09 M, I had to remove the wood trim above the start button and along the passenger dash area. Then remove the main center switch (A/C, NAV,etc). Then pull out the plastic trim around the screen and lift the center speaker cover.