When you have bad credit, car dealers frequently want down payments for car loans for a number of reasons. Sometimes it's the lender that wants a down payment as part of what is called a 'stipulation'.
I'll explain to you how this works and give you a practical example.
When you fill out a credit application and to try to get approved for a car, the finance manager (at one time was me) would then fill out the rest of the credit application with the information about the car that you wanted to buy. This would include the vin number, make, model, mileage and all that jazz.
What happens then, is that the credit application is received by the lender. In some cases, the application may be submitted to numerous lenders to not only see what you could get approved for, but also how much money can be made on your car loan with each particular lender. (This is why when you have even good credit, your application gets sent to multiple lenders by the dealer.) Each lender then replies with a response.
Based on a vehicle that you have taken a look at or wanting to buy, the lender may approve you for financing for that particular vehicle, but add the stipulation that they will only finance a particular amount for that particular vehicle. This then places the burden on the dealership of doing one of two things... either lower the price of the car to the amount that the lender will allow or the dealer has to request that you make up the difference.
While supply and demand can dictate that some vehicles will require them to be sold at a higher price and far above loan value, there are a lot of great deals on both used cars and new cars right now. Finding a good price on a vehicle is the key to eliminating the need for a down payment.
One of the easiest ways to get approved and get the best deal on secondary financing (bad credit car loans) is to compare offers online. There are a few good sources that allow you to do this.