Rent your car at least 6 months before travel online. All agencies are government owned and best is to book via 3rd party in Europe.
One friend of mine used Cuba Travel network, but on arrival they didn’t have their booked 2 cars so they gave them a private mini bus with a designated driver.
Another friend of a friend used Enjoy Cuba and they didn’t have any problems, though by the looks a bit more expensive than other websites and the additional taxes were not specified so not clear how much you have to pay.
Initially we were thinking to use Exelencias Travel, but the website looked a bit unprofessional and when we tried too book, we actually just requested the car and didn’t book it. We had to wait for them to get back. 14 days was only 1100$, the lowest of all four.
In we end we used Cubacation website. In the end we h. ad to request, which was around 1300EUR, but once they got back they sad they didn’t have the car we requested and we ended up paying around 1750EUR (1560 GBP). They’ve sent the voucher with a code to get the car at Rex Office (one of the official Cuban rentals).
We’ve tried to get directly from Rex or other official car rentals, but we had to wait for response and it seemed more expensive anyway.
I think most of those agencies are based in Spain.