The threads are 12mm X 1.5. The bolt head has a radius which is apparently unique to Mercedes. I converted to wheel studs from the bolts. Tire Rack has the studs in two lengths.
If you are using spacers, you will need the longer length. I use 5 mm spacers on the rear for some tire/wheel combinations so I used the longer studs on the rear. In order to use the stock center caps, I shortened the longer studs.

If you want to stick with the wheel bolts, I am sure that Tire Rack can help you there too. Contact: Luke@tirerack.com or call him at 877-522-8473 Ext. 362. Luke seems to be quite knowledgeable about our cars and posts frequently on some of the Mercedes boards.

By the way, I autocross the Crossfire and change tires frequently. Wheel studs make the process much quicker. It is much easier to line up the holes to a stud than to try to balance the heavy tire wheel combination while you try to find the hole for the bolt. It saves a lot of time and effort when you change tires several times a day.