2003 Duramax - Turn Signal / Marker Lights Electrical Issue

Having a small electrical issue with my 2003 Duramax. With the parking lights turned on, the turn signal and side marker lamp alternate flashing. With the parking lights turned off, the turn signal and marker lamp flash together. All 4 corners (left, right, front, rear) behave this way.
I'll update the video if / when I find the problem.

[@Atk876]

Hey bud did you ever figure this out? Does your right side do the same thing? Thanks for the vid

[@sneaky98gt]

As it turns out (no pun intended), that's actually how it's wired up from GM. It's kinda weird, and I'm not sure why they did it that way. The 'sidemarker' lamp is wired in a way such that it doesn't have a dedicated ground, and actually grounds back through one of the 'parking' or 'turn' filaments, depending whether or not the parking lights are on. This is what causes the alternating flashing when the parking lights are on.
Technically, you could wire the sidemarker with it's own dedicated ground, which would result in it doing the same thing as the parking light (i.e. always on or off, no flashing), or it would result in always flashing with the turn (but off when parking lights are on). Just depends which side of the bulb you ground.

[@sneaky98gt]

Oh, and yes, all 4 corners do the exact same thing.