Ive been looking for a good street map for Santiago... and I found it! It's called Mapa Turistico, Santago/Region Norte edicion 2004. I found it at the Sosua Caribe Tours station for 150 pesos. Ive seen a pdf version of some of the detail, but this is a really good map to keep in the car. It also has detailed sections for Puerto Plata, Sosua, Rio San Juan, Las Terrenas, Samana, Bonao, La Vega, Moca, and San Fran de Macoris. They even have points of interest, banks, hotels and shopping spots pointed out in each city.