I have the same problem on my 95 SL. Started immediately after I pulled the gauge cluster to replace some bulbs and disconnected the cluster harness to completely remove the cluster.
I have heard that a bad seatbelt switch can cause the light to come on, but your best bet is to pull codes. That's what I will eventually do when I get time...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car