side marker lights

[camarohair]

hi folks,

let me start by saying that i'm not at all good with electrical trouble-shooting, but am not afraid to try! I've read up on some other threads and am not sure that i've found the answer to my problem.

i've got a '68 C10 that someone set-up with working side marker lights. The front driver side marker light won't come on solid with the parking or headlights (the rest do), but the side marker turn signal does work whether or not the headlights/parking lights are on or off. every other light works as it should, including turn signals all around, it's just the one light that won't come on with the headlights or parking lights. i've replaced the bulb and the entire housing/socket. what steps should i take to figure this out?

thanks in advance for the assist!