I believe the plug is only on the Drivers seat belt buckle. At least my 98 and every other 98-2002 Rangers I've worked on never had one on the passenger seat. The 04+ Rangers (might be some earlier years I'm not sure when it started exactly) have seat belt pre tensioners in both seats, those plugs are known for having bad connections.
I would check and make sure the bulb in the passenger air bag turn off switch thing is not blown. When that bulb quits working it also sets off the air bag light.
Here's a topic talking about it. Look if your airbag light is on, this could be why!