I would not advise doing this job unless you are a master DIYer. I would advise most techs not to do this job unless they planned on doing others as the pitfalls outweigh the advantages even in money. Cracked dashes, cracked windshields, vacuum problems, electrical problems, they can all be handled. I know because my first job back after my first MB school in 1974 was to reinstall the dash that the just quitting senior tech had scattered about his stall.
I would advise the simple task of pulling the blower motor. It is located above the evaporator and is easily accessable for the engine compartment. Its possible that yoo might get far enough in that way. Pulling the motor allows access to the top of the evap. No easy access to either side of the heater core.
Bosch Master, ASE Master, L1
33 years MB technician