I have found the following information on this website - http://www.getspain-spain.com/driving_in_spain.html
Laws and regulations about child car seats in Spain
Please ensure that the car seat is EU-CE approved.
Basically children must be carried to ensure the maximum chance of survival in the event of a serious accident or when a vehicle is forced to make a sudden emergency stop. With this in mind all people in vehicles up to 9 seats including the driver, are required to wear an approved seat belt if fiited and in the case of children, approved seats specially designed for the purpose.
What are the requirements?
Up to 9 months and up to 10 kg, a carrycot must be used laid across the rear seat and secured with the car's safety belt or any specially approved attachment.
From 10kg to 13kg and up to 2 years of age, a child's car seat fitted in the rear with the child facing rearwards.
From 9kg to 18kg and from 9 months to 3 years old, child seats facing frontwards or rearwards , fitted in the rear of the car and secured as detailed in the seats instructions.
From 15kg to 25kg and 3 to 6 years of age, use a seat adaptor (elevator) to lift the child up to fit comfortably with, if necessary, a CE-EU approved seat belt adaptor.
From 22kg to 36kg and 6 to 12 years of age, use a seat elevator so that the child is comfortably protected using the seat belts.
Children up to the age of 12 are not allowed to sit in the front seat.
Unfortunately it does not clarify the situation for a car with no back seats.
Noreen