You can barely see them in this picture, but for the big trucks for something like 40 & 41 they made bases for the headlamps with lights in them. I'll keep trying to find a better picture, but these would be perfect for turn signals.
https://upload.wikimedia.org/wikiped...6234466%29.jpg
Here they are
https://www.ebay.com/itm/32520565206...hoCoVQQAvD_BwE