New Auto Contract/voluntary repossession

So, I bought a new car in Jan. '08 (I was upsidedown) and paid the debt down to the price it originally sold for. The manufacturer sold it for 'Blue-Book' value. They (GMAC) want me to pay $7,303. to them. I don't have it or would not have taken it back.
How can I avoid or considerably reduce the amount of the debt ?
Brian `;o)-