Saving for a down payment or trading in a vehicle can reduce the amount you need to finance and reduce your financing costs. In some cases, your trade-in vehicle will take care of the down payment on your new vehicle.
If you owe more on your vehicle than its market value, you have negative equity in your vehicle. This is a consideration if you plan to use your vehicle as a trade-in. The longer your new credit contract, the longer it will be before you have positive equity in the new vehicle — that is, before it is worth more than you owe. If you have negative equity, you may need to make a bigger down payment. Or the dealer may offer to include the negative equity in your new finance contract by increasing the amount financed to include the amount you still owe on your current vehicle. This will increase your monthly payments on the new contract in two ways: it adds to the amount financed and increases the finance charge. If you have negative equity in your vehicle, consider paying down the debt before you buy another vehicle. If you use the vehicle for a trade-in, ask how the negative equity affects your new credit obligation.
See this article at https://www.consumer.ftc.gov/articles/0056-understanding-vehicle-financing#federal