If it is like my 86 300e, you can try to adjusting the screw behind the center screen. If that does not work, it is the vacuum actuator?.The entire dash has to come out. Been there, done that. If you have the manuals,have the time and patience you can do it yourself and save some money. Otherwise, take it to a mechanic. BTW, while you're at it, change all the vacuum actuators (total of 6 or 7, my had 7) so you do not have to do it again for the next 7 years ;^)
J.Hidalgo