I don't have the CD or service manual, but when you say "seatbelt light", do you mean the SRS light?
If so, it is usually the switch in the seatbelt latch (either side), or the contact ring in the steering wheel.
This site has a huge archive. You can spend hours and hours researching your problems with the search function (if you have the time and the patience).
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".