Since you have one working driving light and one questionable driving light I would take a volt-ohm meter and compare the readings between the two.
I would measure the voltage at both sockets as well as the ground resistance at both sockets. Also try switching the bulbs.
Check the condition of the wires going to the sockets as well as the metal part of the sockets.
Hopefully you will find the problem.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles