My advise is to treat the new/used car purchase, the trade in, and the financing all as 3 separate transactions. Do not combine them. You work your deal on the car/truck you want and negotiate that to the price you are willing to pay. Then work on the trade value. Financing is the last part- you can listen to their options, but go to your local credit union and know your options there as well. Run the #'s , sometimes its cheaper to forgo the instant rebate incentives and take the 0% offers, sometimes the math works out to take the rebates and use a local lender for 1.99%,etc.
IMO, The best route is to take the rebates, and use a home equity line to purchase the car. You get the rebates, you get the right off some of the home equity interest on your taxes, and you have the title in hand. That is what a lot of my customers do on new car purchases.
New car purchases I can't save you that much, If you want used, shoot me a PM