Pete 579 no left turn signal on trailer

They actually sell a tester that plugs right into the trailer socket. Here is one. And your very welcome!

 

Ok, you need.ro see if there is power to the back of the socket on your truck. If there is not. It's could be the wire to the socket. Does your turn signal work on.the back of the tractor?

 

Ok, I would check the back of the socket tomorrow and start there.

 

On my Volvo, I have separate fuses for trailer lights. And, separate fuses for both right and left sides. I had the same basic deal, no trailer turn signal or 4 way signal on the right side. Had them on left side and all the tractor lights. It was the fuse. Replaced it and everything's been fine since.
Unsure if a Pete will have a similar set up, but it seems likely to me.
Or, you could simply not make any left hand turns!!! (o:

 

I have a Pete 387, quite a few years older, but it may help. Mine has a separate relays box under the cab. Once a relay got hung up and the turn signal was on solid. On the trailer only. A new relay solved the problem.
It takes two bolts to open it.