What's the red light in the left wing of a N.A. T-6 for?
It's the passing light. They're no longer in use.

In an era of virtually zero radar air traffic control, two planes headed for each other at high speed at night needed those [very bright] lights. They would use the right-of-way rule to stay on the 'right lane' and safely pass each other.

(jetphotos.com) Clearer view for D-FUKK.



(Code of Federal Regulations) Points (a) and (c) match the T-6.


  a strong red light to warn passing airplanes displayed usually in the leading edge on an airplane flying after dark (merriam-webster).


(Source) Early Connie's had them in the nose (b).