1999 Mercedes Benz ML320 my AC runs hot

With the manual control system you have, failure of certain components will cause it to default to full hot.
A/C contol head- which you replaced
inside temp sensor, next to comand unit
a/c stepper motor controller mounted to heater case
Temp sensor on heater case behind glove box.

The other possibility is that the blend air door is not functioning, I have seen the linkage from the position motor break.

Probably best left to the dealer to diagnose since they have the luxury of substituting in parts.

This system gives little or no diagnostic feedback even with SDS.

Hope this helps.