Though there is already an excellent post that covers much of this, there still seems to be some confusion about the stock bose audio system and how to install an aftermarket head unit. First, a few notes about the bose system and what to look for when buying an aftermarket hu (head unit).
The bose system is a bit wanky and there is only so much you can do with it. It is a power amp that drives all the speakers in your car. The input will only accept left and right signal feeds. This means you can't have front and rear fade controls. You're stuck with fairly anemic subs and an underpowered system in general. To substatially upgrade would mean removing speakers and the bose power system and starting from scratch.
This doesn't mean an new head unit wont improve the sound at all and adding bluetooth is certainly a feature most of us want. When shopping for a new HU, don't worry about buying a really powerful unit as you are only using the pre-amp to feed the hu. It is important that you buy one that puts out 4 volts (some are two) through the rca outputs as that's what you'll be using. Don't worry about 100 watts per channel as you wont be using any of that. Your just using the 4 volt outputs to drive the bose.
I used crutchfiled as they are price competitive and all the extra install gear needed was really inexpensive. I had some tehnical questions and they know their stuff. So when shopping, look for 4 volt output and the features you want (variable display color, bluetooth, usb charging etc). Now onto the install.
Here is the install kit that came from crutchfield. The cool star thingies are to remove your factory HU.
Here's where the star thinigies go. Typical benz. When the rest of the world uses side clips, Benz goes from the bottom. Anyway stick em in and it will release the hu and you pull it right out.
You'll see all the various harnesses for telephone, cd changer etc. You'll only use the one pictured here and the antena lead.
Now, let's go to the kitchen, heat up the soldering iron, open a beer (or thinking fluid as I call it) and get to work. The two rca plugs (white and grey) go into the jacks on the back. Make sure you plug them into the left and right jacks on your new hu and not the rears. You'll only attach five wires from the hu wiring harness to the adapter that crutchfield supplies. They power up the head unit and sends power to the bose power amp. All the signal to the bose is provided by the rca jacks. The are all color coded. I soldered mine and taped them to insulate them though if you're really groovy you can use heat shrink.
This is the antena lead adapter which also came from crutchfield. You'll need this so you can use the antenna in the car. Once again, MB can't conform to most of the rest of the world but what the heck, it was about 20 cents.
This is where the adapter goes on your hu.
Now back to the car and lets test it out. The factory harness is plenty long enough that you don't have to install it to make sure it works. This is the point where you make sure it works. If you've done everything properly, a smile will show on your face and you may reward yourself with more thinking fluid as you're nearly done.
Now, unplug the HU and lets install the din cage. It's the little metal bracket that will hold your new hu in place. Just push it into the opening and you'll here it click into place. Re attach the wiring harness, the rca leads and the antenna and push the hu into its new home until you hear it click into place.
Voila. I hope this was helpful and you see how easy this is to do. The only tools you need are a wire stripper and a soldering iron. If you don't know how to solder, you can use those twisty things. Wire strippers are easy though practice on some throw away wire first until you get the hang of it. It took me 5 times as long to resize these pics and write this post as it did to install the new hu. Cheers and happy motoring.