The airbag is fastened by two (torque?) screws from the rear of the steering wheel. Release these screws, disconnect the airbag and remove it.
The heater cores of (european) W201's are known for an occasionally leakage. I would start check pressurizing the system first!
Tips: remove steering wheel, center console, dash cluster, switches, radio, glove compartment box, side ventilation channels, mid ventilation grating, speaker gratings, gratings and panels under the windscreen to release the two nuts that hold the dash (if I remember correctly).
Changing the heater core can easily be done within a day and even within a half day if you know what you're doing and having a very good day. I always see repairs like that as a terrific challenge...
with a minor 600+k