problem
A blank CBU, a blower that only works on 4, or flaps that sneeze behind the dash.
Engine-bay fuse 7. Then the CBU itself — the LCD and the buttons die together on tired units. Rebuilt panels exist.
The regulator (resistor pack) under the passenger cowl. Classic Mercedes, every chassis. Easy part, awkward clip.
Compressor clutch is fuse A on many cards, plus the pressure switches. The aux fan should cycle; if it never does, look at the same fuse 7 family and the fan relay.