This was a common problem and you may have a special policy or recall from gm.I can tell you what happens,go under the hood,remove the black plastic cover att. to where the hood closes.Go the the wiper motor.Where the motor has a crank arm on it and all the linkages meat is called the park arm.Below that on the metal mounting plate is a little tang of metal that the park arm needs to catch to park,it will be bent down.The fix is to install a new plate that is reinforced. Any GM dealer can check to see if you qualify for this repair.
