If the seller agreed to undertake certain repairs but did not do all of them, and the omitted repair is something material or of consequence (not like changing the dangling air freshener), then the seller has breached the sales contract. Such a breach of contract does not make the entire contract null and void -- rather it may entitle you to damages if you have to sue to enforce the contract. You should write to the seller by certified mail and provide notice of the missing repair and whatever other complaint you may have about the car, and request that the seller resolve your complaints. You should also advise the seller that if the seller does not honor its contractual commitments, you will hold the seller responsible for the cost of repairs if you have to go to another repair shop.
If the cost of the repairs is less than $10,000, and the seller does not resolve your complaints, and you incur the cost to repair the car, you can sue in small claims court to recover for the repairs. A motor vehicle dealer is required to secure a bond to ensure its statutory obligations. If necessary, you may be able to make claim on the seller's bond to recover the repair costs. For more information on motor vehicle bonds, you can check out this web site: http://www.southcoastsurety.com/motor-vehicle-dealer-bonds.htm
It is a breach of contract on thier part if the term that that they did not honor is what is called a "Material term." if it is something like a knob on the radio, i wouldn't think this is material. You can sue for anything, but the question is, would it be cost effective? Many lawyers will give you a free consultation. Find one in your area and show him the contract.
This posting is not a solicitation nor is it legal advice. It is merely any informed opinion and does not create an attorney client relationship.