Possible ground issue

I had a similar issue on a Star. Ground in the turn signal on fender was corroded, backfeeding power down the turn signal circuit. Another spot I would be certain to check would be where the harness runs up to the hood. Every time the hood is open, that wiring gets flexed.

This is an excellent suggestion. Process of elimination. Most likely places to find the issue is where both turn signal and tail/clearance light wiring exists.

 

That's the worst! Correcting custom electrical work is extremely tedious work.

 

As soon as I see "custom" wiring on a circuit I have issues with, I focus in on that. 90% of the time that's the issue.