The M103 has a drain plug on the right/US passenger side under the exhaust manifolds. There's also a drain at the bottom of the radiator. It might help drain the heater core if you remove the monovalve. I don't know, it just occured to me...

Alon, is your mechanic a disciple of Rube Goldberg?

Sixto
91 300SE
87 300SDL
81 300SD