It's not a good idea. I knew it wasn't. I went to the Mazda dealership. Drove a new 6. Loved it. Told the guy I was too upside down on my car (somewhere in the neighborhood of 10,000 or so due to a trade in that I never should have done.) He didn't care. Wanted to make a deal anyway. I told him it as impossible (so far they haven't been able to prove me wrong.
Every few hours I get a call. Well how about this!? Nope. Everything they try is a dead end. They keep trying though (for some reason. No idea why) because my credit is amazeballs. How about a 84 month payment plan? How about no. Some dealers just will not give up. That's the car above.
I expect they will keep trying. The reality is, they would have to give me 20,000 for a car that is barely "worth" 17,500. And that's being generous. I wish them the best of luck, however, at this point they are just annoying the shit out of me with phone call after phone call.