This is a unfortunate situation. My 420 SEL has the same problem. One day while driving, the light came on and never went out. The dealer said it was the control unit (crash sensor) and that the code would not reset. I had the steering wheel taken apart and that was fine, but no matter what the control unit code would not go away and they said it is $1100 just for the part.
I have never seen the part, but I think that price is rediculous.
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)