its something to deal with the air bag system. Its kinda like a default alarm if say the bulb is burnt out or has been removed, its very annoying, both my rangers and the expedition do it
someone told me once that there is a black and white wire in the pass. kickpanel, I could not find it, but said to cut the wire(s) to stop the alarm. Or if you could were the dinger is at, disassemble it!