Just an idea...pull out the other bolds, remove thermostate, you might have a bit of the broken bolt sticking up above the houseing. make sure engine is warm so bolt comes off easier then take a vice grip on top of broken bolt and try to turn it. If that fails the only things I can think of is to drill it out and re tap the threads
2000 C230 Kompressor
1988 190e 2.3 - 225K miles, owned for 7 years. I cannot say enough good things about this car. Very well built, even at 225 it ran like new.