I happened to look at the ACC to manual climate control DIY on this site and noticed one pic that shows the total length of the harness for the blower regulator (bear in mind what I am seeing is the little stub end from the firewall grommet to the regulator, as you can see in the pic).
Are we sure the dash has to come out to reach that plug? It's certainly long enough to not be buried behind the heater box, though I realize that doesn't mean Mercedes didn't do it....