Unfortunately the early systems don't have self-diagnostics. They will need to be tested with the MB tester.
If the light is on, the control unit isn't passing the self test. All the circuits can be hand tested and if they are good that means the control unit is bad. The control unit has to be replaced if the bag has been deployed as the crash parameters are held in the early control units. They can't be reset even with dealer tools.
On 1988 on vehicles the problem could be verified by retrieving the codes. A code ten indicates the bag or seatr belt deployment and tells the tech its time for a new box. All other codes are resetable if the problem is fixed.
At some point in the 90's the control units became programmable by the HHT and the replacement units have to be version coded for the number and type of bags and can be reset in most cases a certain number of times; seems like three but since I have never done it I can't be sure.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician