A lot is going to depend on circumstances (and I know nothing of Spanish law)
If, in the UK, you bought a car with finance, there are two separate contracts, one for the purchase of the car, and one with the finance company. If the finance company repossess the car then it is to cover the outstanding debt, and as long as the car is worth more than the debt then you should be able to stop paying. But if the car (Due to accidental damage etc.,) is worth less than the outstanding debt then you would still owe the finance company some money.
If the car were repossessed by the police, as the seller didn't actually own the car (a 'ringer' or whatever) then you would have a claim against the person who sold you the car (if you can find them) but you would still have a debt to the finance company.
I don't know how this links to your Spanish probm, I'm just pointing out that something similar could happen in the UK.