Heat,
I don't know D14, but the way I found out which roads are car accessible in D16 was to get a topo map and go for a hike... A lot of hikes. Google earth and other sources of aerial photographs also help by giving you a bird's eye view. Get out there, drive till the road ends or you think you might get stuck, and then explore further on foot. Look for sign (I'm not talking about the yellow "deer crossing" sign, but the brown varieties: hoof prints and droppings). After you have done some exploring on your own, report back here what you have found. That may earn you some hints from those who know the area.
Don't forget to take plenty of water and some food, and take along a compass and basic first aid.
Have fun out there. For me, exploring San Diego's backcountry has been one of the great benefits of hunting here. Enjoy the journey!