Try it with and without anybody on the passenger side. If I put the key in and turn it on and if the seat belt light is on solid than the light will turn off after in put on the seat belt. But the light will blink after driving. It will brink on for a while than off for awhile and on again. Sometime I can drive for half an hour than it will goes on again. Sometime it stay on blinking and doesn't goes off.
If I turn on the Ignition and it blink right away it will blink for awhile on and off.