It is as safe as any airport is in the middle of the night. As I wouldn.t get into a car with a stranger in any US city at that hour, I wouldn't do it here either. Many DR1 posters use JJ for transportation from Santiago. He can be e-mailed at:
palomalinda0890@hotmail.com . He is dependable and speaks English. If Sosua is your destination, you can head right out from the airport, stop for breakfast on the way, and get to Sosua when things are beginning to wake up.