Lots of potential culprits when the clutch will not engage.
Unlike the W123 cars, the W124 belt is a serpentine and MB has included a disable on the compressor clutch when the rpm of the compressor does not match the crankshaft. (clutch slip)
The RPM sensor is located on the back of the compressor.
You may also have a KLIMA relay problem, and I recall this is integrated into the OVP relay on late w124 chassis.
Could also have a bad pushbutton unit.
May be low on freon.
The factory manual has an extensive diagnostic section, which is most helpful on these cars. (Hint
)
Jim