From Cabrera, head West towards Cabarete but turn off at Sabaneta to go towards Moca. Go over the mountains to Moca (not Santiago), then La Vega, then Villa Altagracia, to outskirts of Santo Domingo and then head west towards Bani to Barahona.
There is a short jog you can take out of Piedra Blanca south over the hills that will eventually dump you onto Carratera Sanchez (the southern road, route 2) but I do not recommend it. Stay to the major roads so you have some opportunity for assistance if you get a flat tire or some other issue.