I believe that you will need connectors/leads however my local Audio Installer was unable to find the right ones in his catalogues so could be a problem. Search other forum chains on here tho.
That link above was the website i used for the adapters and leads etc. They detail the exact periods of production for each model so i found it quite easy to find the leads i needed for the steering control harness.
You dont need any complete wiring harnesses for the speakers and power etc as the merc has ISO connections so should just plug straight into the kenwoods adapter that comes with the headunit.
You might need to get an aerial adapter for the radio to work though as i found the connections were different but these can be picked up from local automotive shops (i got mine from Autosave not sure if they have stores near you). Just plugs into existing cable then into back of stereo.
Also if you have the bose stereo system installed in your motor you would have to re-wire the entire car for all the speakers etc as i think it has an amp in the boot and uses fibre optics for all the connections.
Also if you have the 6cd changer in the glove box you will lose the use of this as it works using fibre optic leads regardless of bose installation or not. I didnt find that much of an issue as i stick all my music on a dvd as mp3 files so rarely need to change the cd lol.
BMW F11 Touring + Porsche 911 Carrera S + Toyota Alphard Camper
I would definitely go with the latest Pioneer AVIC F series nav units, the 710 or the 910, if you can afford it the F10.
The navigation is is using the latest iGo 8 software which is way ahead of the competition when it comes to routing, it has Parrot bluetooth built in, and the graphics are so much nicer than the other units, plus you can set the illumination to one of 32k colours to match the rest of the interior lighting.
You can add an address in google maps and then send it to your car as well, you can also put an address in google maps on your iphone and send them over to the car as well, decent warner speed camera database too.
I am using the AVIC-F500BT in my 1 series, which is an add on to the factory nav, and I love it.