This problem now solved! -Cleaned battery terminals + took out fuse No.9 and cleaned same. Not sure which of these two things worked, but one did and now the seat belt reminder light operates normally and goes out as it should after a short interval.
It depends on the model and engine displacement. My W124 220E has a 70 liter fuel tank capacity.
By the Pratt & Whitney PW-220E afterburning turbofan.
I had a similar problem on 220E , under the backseat is a box with circuit board, on the circuit board are 8 contactors,,, clean the contactors, I used a small peice of wet and dry paper,, easier if you remove the board from the car.