The two problem areas in the vacuum system for the heat are the vac line that travels under the battery tray. (It gets acid damage and leaks). And also the line that connects to the intake under the air filter housing. Also there is a valve located under the black plasic panel under the hood. Take off the panel and look under the windsheild there will be a valve with a vacuum line connected to it. This is the heat valve and should have vacuum going to it to turn off the heat.
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM