Driving to Isabela, Puerto Rico from Aguadilla and San Juan.

There are major international car rental companies on Puerto Rico - including Avis, Hertz, Dollar, Budget, Thrifty, etc.
There are also many local companies, and one of our favorites that seems to have great rates all the time is Charlie Car Rental.

All of the rental companies are either located right at the airport – or provide free shuttle service to their nearby facilities.

Car rental rates are a bit like airfare - they go up with demand, and as you get closer to the time of your trip - so you will want to make a car reservation as soon as you have arranged your flight. If you find a cheaper deal later - you can always cancel one reservation and make another.

DON'T wait until you arrive in Puerto Rico assuming you will find a good deal on a car rental at the airport.