After this initial period they must refrain from using the car and apply to have Spanish national plates for it. Effectively, they must import the vehicle into the country.
In the case of UK registered cars a certificate of Permanent Export (V561) must be obtained from DVLA on surrender of the UK registartion document.
Also a Spanish road worthiness certificate (MOT) will be required from an ITV centre, do not forget to have your headlight adjusted to comply with EU regulations, prior to the ITV inspection.
There is a special tax (impuesto Especial) to be paid, currently based on 13% of the ready-reckoned value of the car.
While the documentation is being processed, applicants will be issued with a temporary set of documents and temporary number on green plates.
Non residents are not permitted to use their foreign number plated car for a period exceeding 6 months in any one year, the best way to save any arguments about this is to apply for a temporary tourist plates, as it is rather difficult to try and prove that the car was only used for 6 months and garaged for the other six. Temporary tourist plates are valid for 6 months and must be renewed each year.
BEWARE the police have been rather laid back on these points in the past, but they are getting keener to prove the point. The fine can be quite severe, you may lose the car in the bargain and have a heap of problems.
(the above information is purely informative, please consult a professional for personal advice)