The only way is to convince them that you cannot pay. Tell them you are now unemployed. Make them beleive they will get nothing. Get them to take the car back as full an final settlement. They will whine but in the end they will prefer that to having to reposess it later. Draw up a letter referring to the car and get them to sign saying they take possession. The key phrase is "in full and final settlement" make sure the car is mentioned by reg number take copies of all documents.
When I hear questions like this I wonder how they sell new cars to private punters. They are soooo expensive and you lose half your money within 1 year in most cases. In fact I'm going to post.............



