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Seatbelt retractor on front passenger side, if you pull the plastic off the door post you will see it.
Check to make sure it is still plugged in and not damaged, little 2 wire plug for the sensor for the SRS system. If the plug is not the culprit then start checking the wires for resistance until you find the problem. Only the driver and passenger first row has the sensors on the retractor side (at least 2000-2003 model years, I would assume '04/'05 as well but I have not personally messed with that on the 2 later models).
So at a glance either somehow the sensor went bad, or a wire/connection got broken going to the sensor.
I had put new retractors (the seatbelts) on the front seats last year on my '01 Ex, if it was in an accident you will need to have it rebuilt or find one in a boneyard. Rebuilt was something like $200-$300 each. I got a full set, for all three rows, from a local boneyard for $100.