Not sure if the double '71 housings are the same as the single '70s but I had trouble at different times with the socket losing it's ground and thus losing the lights. On the '70 housings, there is no ground wire to the socket. The socket is crimped onto the housing and is grounded through the housing to the bumper.
I tested it by taking off the lens and holding a jumper wire against the socket beside the bulb and then touched the bumper. Lo and behold, the lamp lit up. I pried up the endges of the socket it a couple spots and scraped both the underside of the socket and the housing and then tapped them back together with a small drift. This worked for a couple months and then I lost my ground again. I did this a couple times on both sides.
I finally fixed it by taking my Dremel and making a small shallow groove in the backside of each socket and soldered a wire to it. I soldered a terminal ring to the other end and ran one of the housing bolts to the bumper through it. Now the socket has a hard wired ground and has given me no trouble for 4 years now. Mine is a daily driver so I was more concerned with safety than factory correctiveness. You can't see the wires even if you try because they are up behind the bumper on top of the housing.
