How can I replace a seat belt sensor? - 1999 Toyota RAV4

The sensor is between the springs under each seat. The seat belt itself is not the problem.

Disconnect the battery negative terminal.
1. Wait at least 90 seconds after the ignition switch is
turned off and the negative (-) terminal cable is
disconnected from the battery before starting the
operation.
(The SRS is equipped with a back-up power source, so
if work is started within 90 seconds of disconnecting
the negative (-) terminal cable of the battery, the SRS
may be deployed).
2. Do not directly expose the steering pad, front
passenger airbag assembly, center airbag sensor
assembly, front airbag sensor, front seat inner belt
assembly, seat position sensor, occupant
classification ECU, front seat side airbag assembly,
side airbag sensor, curtain shield airbag assembly,
rear airbag sensor or front seat outer belt assembly to
hot air or flames.

check to see if the wire has came loose leading to the seat belt disconnect button you push when releasing the seat belt if connected. then will have to replace that part of your seat belt the sensor or switch in buckle may have broke

seat belt sensor or air bag sensor, some cars measure weight in the seat on the passenger side and if it is below a certain amount it disables the passenger air bag, a safety for little kids, other sensor for seat belts, if there is any weight and no seat belt is fastened it will turn the seat belt warning light on in the dash.

The weight sensor in the seat is bad if this is the passenger seat. When you connect the seat belt, the sensor is disabled.

There is a sensor right at the bottom of the seat belt, that sensor works hand In hand with the airbag sensor. The solution is to replace the whole belt

faulty seat belt sensor- switch. Check the wires running beneath the driver's chair if shaved or disconnected. If ok, replace the seat belt lcoking mechanism.

If the seat belt has jammed, you may just need to replace the mechanism- you want it to do its job correctly when you need it
For your information,--You may need to replace the mechanism for the seat belt. If the impact restraint system has encountered what they refer to as a stage one deployment due to a minor accident, the seat belt locks up, and even though the airbag(s) hasn't deployed, there are frame sensors that may need replacing reset.
We encountered this with my daughter's 05 Focus and needed to replace the belt assembly, and a couple sensors on the frame..

There is a sensor that makes a connection when the seat belt is buckled, that turn out the light, it is defective, replace the seat belt assembly or remove the bulb.