The circuit grounds through the housing/body of car (no ground wires). If the lights just quit working without you doing anything with the housings first, and the bulbs are good, I would next look at the NSS as you have suggested. If you had been working on them before they went kaput, then recheck that the sockets have continuity to the housing and housing to the body. The NSS might not itself be bad - check its connectors and alignment before spending money on a new one.