The above responses are correct-- above 20-30 mph fans are redundant (and may actually slightly impede airflow). However, if your thermostat is defective the motor would most likely overheat regardless of operating conditions.
Yours sounds more like a case of diminished cooling capacity. My first guess would be an externally clogged radiator or condenser. I just replaced the radiator in a '93 300E and I took the opportunity to clean the condenser with compressed air. You wouldn't believe the amount of bugs, dirt and leaves that came out, and this was on a well maintained 97k mile car!
It's also possible that enough deposits have formed inside the radiator to reduce cooling capacity but considering the fact that your air conditioning is also affected I'm betting on the former, especially since the cooling systems on these cars are marginal in high ambient temps.
In order to properly clean the condenser it would be necessary to remove the radiator and blow air backwards through the condenser. It's worth it though, since that's probably the least expensive option.
'93 190 2.6 sportline