My niece has a 2004 Tribute with an air bag light coming on sporadically. She says it mostly comes on when she turns on heater or headlights. She can make a 40 mile round trip without it coming on if she doesn't turn on any thing. That seems like it must be voltage related but I know nothing about the specifics of Mazdas and I live 600 miles from her so cant' do on site troubleshooting. Does anyone have any suggestions about what may cause this issue?

 

Without a diagnostic code scan you are clutching at straws.