These two links from Ken Yee's site are very helpful for installing the stereo.

This link is for how to remove door panel:

http://www.keysolutions.com/M-Class.nsf/8178b1c14b1e9b6b8525624f0062fe9f/5fc50749622a00f48525665f000b0f5f


Good instruction for the stereo install

http://www.keysolutions.com/M-Class.nsf/8178b1c14b1e9b6b8525624f0062fe9f/ad96201628ab6ea685256921000856f8


Positive and negative terminals ought to be written on the speaker terminals when you remove the wire. Just look before you take off the 2 wires.