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be sure the charging system is working properly. i have come across a few that needed an exciter in the altenator and would not charge at idle when started, unless engine is reved up soon as started, light would go out. check this first.
Since you didn't add what year the truck is, it makes it a bit harder to help you....
Anywho...On my 94, the airbag light lit up simply because there were a connection, etc loose, broken, whatever.....The airbag light should be its own little circuit; Dedicated solely for the Airbag....
if i recall correctly there is a small blue box on drivers side under dash, this sends the correct voltage to the ars modual. replace it. some say it has a small battery inside, but, i've never seen one.