(My first thought is If you have an idea where it is leaking is it possible to carefully pry the rubber away, fill the leaking area with Black silicone sealent, wipe off the excess and avoid taking the chance of busting the window alltogeather. The detractor is that dirt sticks to exposed silicone; take care not to leave any exposed.)
If I were going to tackle the job I would invest in the tools needed to do the job. Harbor freight sells (also your local autoparts should) a tool with a T handle attatched to a thin cable and cutter to separate the sealent from the rubber molding. I cracked a windshied (not a Mercedes but I was still trying to be careful) trying to remove a windshield without this type of tool. After you have separated the rubber molding from the sealentand the rubber moulding is really stiff (and you plan to replace it) it might be easier to very carefully cut it to free it from the bead area of the body than to try to pry it out. (See if you can find another web site that has photos of the job being done and what they used to do the job; even if the car is not a Mercedes.) For installation find out if there is something that other folks us as a lube (maybe dish soap and water?) to help ease the rubber molding back in (is sealent used during installation?). Also if it is the rear window has the elcetric defogger you need to be carful of the connections. Whoever change the window on my car befor they bought detatched the copper sheet metal (where the wires attatch) connectors from the window and I do not at this time know how to permenantly attatch them back on to the replacement window (plan ahead). Please let me know if any of this was helpful. Good Luck.