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
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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!