Check the voltage regulator first (cheap and easy). But I think you would have other electrical gremlins as well if this was the problem.

Check the seat belts (crud in the receptacle). Not likely but easy and cheap to check.

Most likely though are the SRS brushes that contact the horn hub in the steering column. Should be lots under a search. Brushes are cheap but the horn hub is about $80. You may notice that the intermittent SRS light is coincident with turning the steering wheel.

glenmore
1991 300CE
1990 LS400