What is it?

On the inside of the trunk on the fender there is some kind of stuff sprayed on for a sound proofing or something, in places it is a light coat and in other places it looks like it is 1/8 inch thick and in strips. Not sure how to explain it really. Not really strips but it looks like the foam you would spray that expands for sealing in a house and they sprayed it on thick (approx 1/8") in places. Anyone know what it is or if you can buy it and what exactly its purpose is?

Hook

Sealer, you'd be amazed at how many places manufacturers actually seal up while the unit body is being welded together. There is a whole manual used at the factory for just spot welds and sealant.

You can also use automotive undercoating. It comes in spray cans and you can get it at any auto parts store.

Hooked,
what you are explaining sounds to me like sound-proofing. It has an orange peel effect and was sprayed on before it was painted. You should be able to find it at any Mustang parts store.

Doug