A lot of people who have had this on various model MBs in the past have mentioned: replacing slip rings in the steering wheel for the air bag contact; checking the battery for low voltage; and checking the seat belts connector and switches to make sure the circuit is complete when buckled. A more thorough search than I performed may turn up more possibilities.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E