Its always funny reading the same old tired nonsolutions on domestic boards
Buckle your belt, put a junk yard buckle into slot, your going to die !!! without it
Well many of us truck users work on jobs very low speed or plow snow slowly and must twist turn to look behind to be safe. and why do we need to be nagged with annoying alarm when a simple flashing light is all you need ? BTW your GM service dept can drastically LOWER VOLUME of alarm more than you can with menu choices on infotainment system. If they say they can't tell them call GM cause it can be done. For those who don't mind being constantly loudly nagged do nothing
Most the other trucks have a simple on off seat belt buckle sequence readily explained on youtube but GM you have to have an advanced OBD scanner and you simply change the audio alarms. you simply turn off the seat belt alarm usually under Dashboard scan. Read up on what scanner will work and how for you.
Let others here know what worked and how