The bulb holder has 3 wires total...2 are brown and one is Grey (on mine)...so I suspect whatever your 3rd wire is would help.

I see this same color on the other (passenger) side fog too...although it too disappears into that whole headlamp assembly.

I appreciate your trying...but if like mine, not much more than "it exists" can be determined without taking things apart...I understand that may not be possible.

Jeff
98 c230
82 Euro 107
69 113
69 TR6
Others