Does anyone know the gauge size of the wire that is the taillight? Because the 2 ends where it's split, the wires don't reach anymore, and I'm trying to figure out a way to fix it. I'm thinking adding wire in the middle and tying it to each end.

 

Not much power going thru those wires, most any wire with an outdoor rating would work, probably a 16 or 18 gage multi strand wire will be plenty.