The default will go against you, strongly. Defaults have a dramatic affect on your credit score and in this economy, lenders are less likely to take such a risk. What you may find is a lender that approves the loan for a very high interest rate. You would be better off saving your money and paying cash. Instead of making payments, take that same money and save it towards the cost of a car. It may not be as nice as desired, but it really lets you know what you really can afford.
As far as correcting your credit, contact the credit card company and start making payments on the amount that was defaulted. If you pay off that balance, it will show on your credit report and have a positive affect on your credit score. Plus the item will be removed in the next 5-7 years thus making your credit history more favorable. Otherwise, the default will remain on your record and make any future loans increasingly difficult to obtain.