"You should have seen what they'd done to it! It was covered with lipstick and they'd tied cans to the bumper."
What would say here "they'd done to my car" or "they did to my car" ?
In the first clause, they seem equally appropriate, but the last is better with "they'd" (IMO). In this sentence it's a stylistic issue.
For example. Say you gave your car to a friend for the weekend. He returns it with a dent. We might say "Hey man! Look what you did to my car"?
You can also use the present perfect here. "Look what you've done to it!" Not the past perfect.