If you currently owe $29k on the Sonata and trade for 17. You're in the hole 12k. If you put 20 down, it would be like you're putting down 8k if you were coming in with no trade.
I would suggest making massive payments toward the Hyundai instead of saving the money. The reason I say that is that you're paying more interest than you'd make saving the money. The only way that wouldn't be true is if you were paying 0 interest on the Hyundai. It also puts a good face on your credit report.
Just my
