My dog won't stay off my car.?

Yes I have a dog that just won't stay off my car. I have a sports car so it's low to the ground for her to get up on. She's been doing it on and off since she was a puppy but it's recently been almost everyday.I've punished her for it and she knows she's not supposed to go up there because when I ask her "Did you go on Mommy's car?" she cowers and put her ears back and her tail between her legs then goes to her cage willingly for timeout. I just don't know what to do. I can't find anything on the web for help. I'm guessing because most people don't have this issue with their dog but rather their cats. If anyone can help to give me some advice I'd really appreciate it. Thanks