Trueog, don't dismiss Manny's advice about taking the car to a good garage.

An experienced MB mechanic through familiarity can hone in on the problem quick and will know how and what to repair.

I also admire your initiative to try and fix the car yourself, you may wish to purchase the Haynes manual for your C-class, it is a British spec book - but it is essential. T-stat replacement is a DIY job if you have complete instructions, which you'll find in the haynes manual, I think Amazon sells it, ISBN: 1859605117