First off, I have an undertail kit that was installed years ago by a buddy and has been working perfectly.

So, at the end of last season my rear right turn signal stopped working out the blue. The left works just fine (front and rear) and all other lights work as well including the front right turn signal.

I have no clue where to start to see where the problem could be. Since it is LED, I'm assuming it is not burnt out. I've jigged all the wires and did all the no brainer stuff.

Where do I begin?

 

in order to put that undertail kit on your friend had to splice in wires.... i would take the undertale kit off to start with and check all the splices and if there is no issue there go to the relays for the turn signals... if that fails make sure there is continuity all the way through each wire

 

go buy a voltmeter and come back here

 

True story. A voltmeter has been my best friend since I've owned a bike.

 

Yep yep. You always get it diagnosed right the first time