Turn signals don’t work when headlights are on

What size wire should I run from the battery negative to the core support ? Thanks for all the support everyone.
I dont know. I don't like addon wires at battery terminals. While at the parts yard (LKQ) I picked a ground cable with the 2nd wire built in. I forget what it was on. Was there to gather various sizes of split loom and other wiring upgrade needs. Retailers ask far to much coin for some things.