it can be pretty ugly especially if you have a manual,

i am dealing with an issue right now where the shopped tripped the airbag light. they are paying to have it taken care of but it is not fun to deal with.

find someone who is experienced in mercedes wiring, drive the extra distance if you have to. make sure it is warranted for life.

you will need a bypass module to tap into the car, xpresskits sells one, or you could use a universal and loose one of your keys.

universal: $50
xpress: $125