70 Dart marker light bulbs - push in and twist to remove. Sounds like the only working ground for your lights was through the one working one, and when that one went you lost the rest of them. The directional sockets in the grill have their own ground wires that lead back to the radiator support. Trace the wires from your broken marker light back and you're liable to find a similar wire back to some sheet metal nut welded into the radiator support or similar. Remove and clean every one you can find - the wire end, the backside of the bolt/washers and any paint on the body nut. I usually make a small jumper from the negative of the battery terminal right to the rad support for good measure. 'In general', when dealing with a stock electrical system, you can't have too many grounds - good, clean, solid ones. Now you modify your system with all sorts of newer electronics and too many grounds can cause other issues.
