Carleen,
If someone is not paying their debt, then it gets to the point where bailiffs are called to try recover the amount owed. A list of valuable things which can pay for the debt is taken into consideration and the easiest thing to take away is a car as it can easily be towed away without having to break into your house. However bailiffs are not allowed to take things that do not belong to the person who is in debt. The situation becomes more complicated when something of value is owned by 2 or more people but still they shouldn't be able to confiscate it (or sell it to get their money back) because of shared ownership.
If you have been a victim of such a situation, then you should call (and write a formal letter as well) the company which has sent the debt collector and tell them of the shared ownership of the car. If they don't return the car, then can should contact a solicitor/debt advisor or the Citizen Advice Bureau to get advice on how to get your car back.