First thing I would do is replace the stock speakers with a good pair of aftermarket ones like Infinity, JBL or MB Quart. Stay away from EBAY off-brands and department store cheapies.
I believe the rear speakers are 4" in your wagon. And the front should be 4" as well.
You might be able to squeeze in a 5" speaker in the rear with some minor mods, but I wouldn't recommend it.
You have basically two choices with any of these brands.
You can go with the standard co-ax 2 way style, or a component system. The main advantage of the component system is that you can mount the tweeter separately, rather than hide it behind the factory grille.
This adds to cleaner "highs" and a choice of where you want to place the tweeter so it can be heard.
The other advantage is with component systems, you get a crossover network that sends the lows to the woofers and the highs to the tweeters.
On some crossovers, there are roll-off switches that let you customize the sound of that speaker to the place it is mounted.
This is mostly useful if you plan on running a sub-woofer, in which case you do not need to send a full-range signal to the speaker, only midrange frequencies.
I also highly recommend you purchase a pair of XTC foam baffles that go behind the speaker (
www.crutchfield.com).
These baffles help in so many ways.
First, it reduces road noise.
Second, keeps moisture away from the speaker, and most importantly, improves the acoustic properties of the actual speaker itself.
As you know, if you pull out your stock speaker, and look into the hole, it is just a big, open "void".
Mercedes did not design this area to acoustically enhance the speaker.
It was just a convenient place for them to mount it.
Raw speaker drivers, of any kind, be it studio, or automotive, are designed to be used within a certain enclosure size.
Most people who install car speakers do not take this into account when they do an install!
This is why I recommend using those XTC foam baffles I talked about earlier.
These will seal the rear of your speaker, giving it tighter bass and also cutting out any road noise that leaks in through that void in the cars body, and will actually make the speaker sound louder.
Lastly is how much power you are giving the speaker itself.
Believe it or not, you can do much more damage to a speaker by under-powering it than by over-powering it.
Make sure your receiver can deliver a decent amount of power, or even better, get a dedicated amp to power the rear speakers.
You don't need billions of watts to drive them.
A good, clean solid amp delivering 30 to 50 watts should do just fine. Just make sure if you do go with the dedicated amp route for the rear, that it is properly grounded to the car, and at least 12g wire is used for the amp's main power.
And, don't forget to shop around.
Good example is Crutchfield wanted over $100 for a pair of Infinity's for my W126.
I ended up finding them on-line for about $60 shipped!!
And, since I have had them, I have had no problems period.
Hope this helps!
Good luck on your project and please let me know how it turns out!