1989 300E

I attempted to address this issue some time ago and have not found a resolution as yet. My seatbelt warning light does not blink but remains steadily on even when the ignition is turned off. I have attempted to do a search in the archives without success. Additionally, I have also attempted to confer with tech manuals etc without finding the possible cause for this. Doe it mean that the unit is defective or perhaps the switch for the door is defective. How would one trouble shoot? Thank you in advance for any feedback.