SRS warning light-trouble shooting

[torkydzl]

SRS warning light in 95 E320 is illluminating on dash.
Seems to illuminate and stay on once someone sits in front passenger seat. After the passenger leaves and I turn the car off for a while and start it up again the SRS light will not turn on until someone sits again in passenger seat.
I have read most of the threads on this issue. Can I assume it's a bad passenger seat belt switch? or do these cars have seat pressure sensors, too?
Followup, is what part do you order to fix this problem?