I have a 93 190E 2.6 with similar problem. Replaced leaking evaporator but still got hot air thru central vents. I froze one of the "inlet doors" and saw temperature dropping and worked fine for a while. When I got hot air coming in again, I placed a valve in the water hose going to the heater pump. Valve can be opened manually at will. As I live in Texas I do not mind having only cold air coming in as climate here is hot almost 10 months. However, such "solution" makes me wonder if it will "damage" something else in the A/C-Haeater system. Any comments are much appreciated.