How late?
And where are you going?
Car rental is not yet ( I think) available at the airport, although I hear they are almost ready.
Santiago has a host of rental agencies with Nelly and Honda as two of the larger ones. Anell in Gurabo, Avis is here, B&H Rent A Car, and Metro Rent a car.
suggestion: If you are not too late, take a taxi to Metro and take a bus to POP....
Lots of options...
HB