There is something called the European Car Rental Conciliation Service (ECRCS) and they have a free service to help with car hire complaints and any other issues with cars hired abroad. (Europe only)
Sadly it only deals with complaints from member companies so most cowboy car hire firms are never likely to sign up to join the scheme. (Read about Car Hire Crooks Here)
That in itself should tell you something about a car hire firm if they are not prepared to sign up for the ECRCS guidelines for treating car hire customers fairly.
The members (correct as of July 2015) are Avis, Alamo, Budget, Europcar, Hertz, National,Sixt, Firefly and Enterprise.
If you're using the ECRCS to help you get your money back, you must have booked with the car hire company directly - not through a broker or travel agent so be careful using Car Hire Comparison Websites to actually make a booking.
The ECRCS will investigate to see if a car rental company has breached its code of practice. The code covers advertising, customer information, vehicle condition, pre and post-rental inspections, and billing.
The ECRCS can get refunds for charges it rules were incorrectly made by a rental company. Visit their website here.
Car Hire Tips
See our tips on Hiring a Car in France or any other European country here.