Car rental company in Spain does not return the deposit (Fråga)

Hi, we are a family of foreign scholars working and living in Sweden now. We went to Spain for the winter holidays and rented a car there. They took 900 Euro deposit because we refused to buy their insurance (had our own). We returned the car in the same condition we got it. The deposit should have been returned max. 24 days after the return of the car. It is almost 2 months now. The company only promises 'to speed up' the process. How can we influence the situation and where to complain?
Thanks!