I bought a new servo for my 1980 450SLC from Randy Steele at epsparts.com. This is an improved version over the factory original. Frequently the factory servo is cracked. The new unit uses an aluminum base rather than plastic. I've forgotten the cost, but I think it was around $180.
However, even after replacing my servo, I don't have heat unless I use the defroster. Additional problems exist in my car due to vacuum leaks. Unless you either correct the leaks or are lucky and don't have any, just replacing the servo (and the control unit under the dash) won't give you heat. Until I get a copy of the ACC manual for my 107, I won't really be able to track down the problem.
1971 300SEL 6.3