This is a tough case. Trying to fix it yourself sounds like something I would do. The thing is, when you go to the dealer, what do they see? A person could have been trying to modify something and damaged it.
A similar situation happened to me when I leased a car and found damage on it. You should inspect everything very very very carefully before you take delivery! (Did I mention very carefully?) This was the lesson I learned. The car is inspected and cleaned before it is given to you, and they can kind of tell because of this if something happened before or after delivery. That being said a busy dealership might miss something.
If you are sure that the damage was there before you took it then bring it back and ask to return it. That is what I did, and I even brought my checkbook. I was really upset because even though I was young, I hadn't had this kind of experience with the other two cars I negotiated leases for. What ended up happening to me was that the salesperson admitted he didn't check for damage under the car and the general manager decided to have it all repaired. They claimed, and validly, that I could have scraped the bottom of it driving it around the approx. 100 miles that I did, and it was a lowered car. But as they saw me as sincere in my beliefs they took care of me. (I bet every time we take the car in for service they are checking the bottom for damage, and hah, sure enough none...)