Right off the bat you need an attorney. While you should have at least consulted one in 2007. You now definatly need one, as there are probably a lot of details that are being left out. When you do be sure to bring EVERYTHING you have from the Divorce Decree to anything on the suits and garnishment orders.
Just some other general comments that may give you things to look into.
First there is no "major signer" on a car loan. You were a co-signer and thus equally responsible for the loan. To be totally honest with you the reason they proably went after you was because you probably make more money to where they could actually collect on any judgement.
What did the divorce decree say? Who was responsible for the car loan. If it states he is fully responsible, you can go after him for the full amount you are having to pay. If it states you are both responsible for half then you could go after him for half. I don't even think you would have to wait until the debt is paid. Now, if it does not say then your attorney probably dropped the ball as that is a major debt that should have been included. Because you don't want this to come back to you saying you were 100% responsible for it.
Once a car is reposessed any deficency balance usually falls under the UCC, which has a 4 year Statute of Limitations. Were you served with the original suit or the garnishment order in 2007? If it was the original suit, then yes most likely it was out of the SOL. Unless your ex had decided to make even a single payment within the last 4 years. So not sure exactly what you can do now.
Now if by chance in 2007 that was the Garnishment Order, were you ever notified of the original suit? If so did you go to court to defend yourself? If you didn't even know you were sued until you got the garnishment order, you need to do some research. This includes contacting the court the original suit was filed in(you can track it down through the garnishment order). From the clerk of the court you are interested mainly in the "Proof of Service". If you can prove you were not served properly you can file a motion to have the judgement vacated. If granted this can remove the garnishment order. Now, they can re-file the suit but you would then get your day in court to defend yourself. Including bringing in items such as if the SOL has expired.
Oh and states take a very dim view of parents who do not pay child support. I bet just the threat of you turning him in to the local DA may lossen his pockets. Because depending on the situation they could revoke his drivers licence, or in some cases put him in jail.
