The most likely problem is your passenger side seat belt wiring or seat belt itself. When your dog or anyone else sits it the truck it activates the occupancy sensor . You truck isn't getting the signal from the seat belt that its latched so it's giving you the warning signal. I would recommend checking the wiring from the seat belt to the switch to see if it somehow got damaged or disconnected. It should be pin#19 LB/GY in the passenger side yellow seat connector. I would check there first. It could also be a problem with the seat belt itself. You could possibly try switching with your old one if you can't find any issues with the wiring. The fact that it only happens when someone is in the truck tells you your occupancy sensor is working correctly and if it was an issue with the seat airbags you should have the airbag warning light on the dash.
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