When you have a lot of negative financial history getting auto financing can be a very difficult task. Getting bad credit car loans and using a cosigner can make things a lot easier for you, but there are still some things to think about here.
First of all, if you have negative financial history then building up some good history with this new loan could help you out a lot in building up to a good score. If you make the payments on time each time this will show future lenders that you were able to handle making those payments and you'll have a much easier time borrowing on your own. However, there are two parts to restoring your rating--fixing your past problems, and building new history. If you haven't done what needs to be done to deal with your past financial issues then that will still be a big problem for you, and should hopefully be dealt with first.
Of course, if you need a car to get places, like work, and have no other way to obtain one, then you'll need to move forward despite this issue.
When using a cosigner they are saying that if you fail to make your payments they will become responsible for them. That means that if you don't pay and then they can't pay their credit will now be damaged. It is very, very important that you are very sure you will be able to handle these payments when using a cosigner.
Depending on your financial history, and theirs, this may be all you need to do to get approval. Of course, if your financial history is particularly bad, and theirs isn't particularly great, you may still need to do more. Having a good sized down payment on the vehicle always helps your situation, as does showing that you (and your cosigner) have stable jobs that pay you well.
With any car loan you take make sure to read the terms and conditions before signing, as there may be fees in there that you'll want to be prepared for.