not sure, but could it be the relay that works the heated seats, or are they on a circuit with something else as well? reason i am asking, i had the rear defrost on a Mustang doing the same thing, the light would come on, but cut right back off. it actually was a timer relay that had burnt out. just my .02.

 

sorry i was never able to determine the cause. tried everything except the last suggestion from sscully. give that a go and post the results? he was very helpful with me.
i have since traded teh truck in and now have a 2010.