GMAC Insurance Benjamin Ham Denied payment for damage to my car caused by their client Beaverton, Oregon

Denied payment for a part that damaged on my car as a result of a collision that was caused by one of their clients. My car had never been rear- ended and has all the original OEM parts on the back bumper as denoted by the VIN tag on each piece. However, the so called GMAC estimator decided that a piece under the bumper had been damaged previously and not by his client. As I said, all the parts were original with VIN tags so they had never been replaced. So to damage this part inside the bumper is like trying to get an egg out without cracking the shell.
I'm extremely disgusted with GMAC and hope I never have to do business with them in the future!!!