This Truck has been sitting for quite some time, also the Turn signals only work on sleeper when the running lights are on. I did replace the turn signal switch and tried installing a LED Flasher but it kicks out immediately. The flashers will only work with non LED flasher unit , also I noticed when the R/R Blinker is on I will see a Dim Flash in the L/R Blinker and this will go away when I pull the R/F blinker Bulb. I have added a Ground to the Frame from the Battery and have cleaned the Frame to Cab Ground.
Does your truck have a bunch of grounds wires going to a bolt up on the inside of the right frame rail near the front of the engine or radiator area. I was having issue one time with 1992 FLD 120 with the Detroit Series 60 shutting down and found the grounds corroded there. Not sure if turn signals are getting the ground there but that might be place to start.
