Ira,
Do you realise that those tee-nuts are fitted from the opposite side to the item being fixed? So to use them for attaching fenders, they are pushed into drilled holes from the inside of the teardrop and bolts would pass through the fender and the body wall. They have some depth too (say, at least the bolt diameter) so they need to be fitted into material at least that thick.
I was getting the suspicion that you have these nuts being fitted from the outside. If not - my apologies.
Andrew