On my 91 Bonneville you have to slide a thin sheet metal like tool down
the slot that the head rest post goes into. It releases some tabs inside
the seat allowing the headrest to be pulled out.
Some models you have to disassamble the seat to remove the headrest. I'm
not sure how yours comes apart, But I've given you a few idea's. An
upholstery shop should be able to advise you.
05 Park Avenue 34,856
91 Bonneville LE 306,078