That applies even if you're getting a deal on the financing, so you can get the same promotional interest rate on the whole value of the loan. You can also get gap insurance to cover the whole value of the loan.

I had the same issue as you and rolled negative equity into a new van purchase and got 0.9% on the whole loan value.

Not recommended to roll negative equity into a new purchase as a rule, but it can be a solution that makes sense sometimes.