I have a 1997.5 Buick Regal GS. When I got the car... both cornering lamps worked. Now, only the passenger side works. I have elimated some causes of the problems:
-the bulb is not burnt out
-the socket is good
-the wiring inside the headlight is good
Anyone have any idea what the cause of this could be? I wonder if it's the turn signal lever... but I'm not sure if the switch for the cornering lamps is in this or not.

I had a similar problem with the driver's side cornering lamp and it was a bad wire contact on the bulb. It was not making good contact in the socket. Bent the wire out a little. If that doesn't work check the 12v with lights on and left signal on with a multimeter. Also check the ground on one side of the lamp socket. If no 12v then there is a high probability the problem is the multifunction switch. Good luck!