Hi,
It will most likely be your passenger seat occupancy sensor, the light can only be turned off by the MB diagnostic machine "STAR", if it was me i would look at getting it hooked up to try to extinguish it then if it doesn,t stay off the software should tell you why.
The seat sensor being the most likely the culprit a new sensor is in the region of £20-30 or £70-80 though i think you will be lucky as i believe yours being a 98 will have the cheaper item.
Alternatively you could try to source a second hand one but removing/refitting of it does require taking the outer cover off of the seat, a garage should charge about an hours labour to do it IIRC.