I bought the truck last year. Changed the bulb recently. It comes on when I pull the light switch all the way out. But doesn’t blink. Doesn’t come on on the first click of light switch with the others.
sounds to me like a switch, ground, or socket issue most likely. The same wire that powers it in park should power it in headlamps(its doing the same thing) and all I can say is you should check your grounds chassis to engine , chassis to bat, bat to engine, chassis to frame. etc
also, you should show us your socket with the bulb removed
Whats your side marker do when you use the turn?
Also the bulkhead connector.. even if it wasnt bad Its worth pulling them and using contact cleaner and new dielectric grease on them. mine was BAD lol. I believe all those functions route through there(I think)