I don't know if different model years have different wiring for the license plate light, but on my 17 I unplugged the existing license plate light, then picked up a matching terminal connector from Cycle Terminal, connected it to the end of the new light wires and plugged directly into the original license plate light plug. As for which is positive and which is negative, I'm pretty sure I just poked the wires in one way, and if it was wrong I swapped them to figure it out. Probably not the best way to check polarity, but it worked for me. The connector is a JST JWPF 2 Pin Connector, but I don't remember if you need male or female for the new light.
Ducati - KTM turn signal connectors