We were able to rent a car at the Santiago Airport from Diagonal rental car. We were met at the airport by Luis who delivered the car to us. But best of all -- we were able to arrange to drop it in Santa Cruz where we would be picking up our tour. We took the optional insurance so the rates were a bit higher than we usually pay. But the ability to drop it 3 hours south of Santiago we perfect for our plans.
We easily drove to the coast, stayed in el quisco at a new B&B then drove to Valparaiso, Vina del Mar, Isla Negro, various wineries in the white wine district (Casablanca), then to Santa Cruz.
One caution -- do get a cell phone number for your contact... our pick up guy was about an hour late due to unplanned road work on the way to Santa Cruz.
Cynde , Bend, Oregon