The SRS light on my 1992 500E has been on for about a year. I finally got around to asking the service shop about it, when the car was in for some other odds and ends, and they called me today and advised me that the problem seemed to be in the airbag control module. According to them, this is the same part that is in other E class cars of the same era. A good used one seems to have fixed it. The part I was glad about is that this is evidently not a part specific to the 500E.
More than likely, I am the last person to find this out, after everyone else already knows it, but I thought it was worth mentioning.