Where is the flasher located??

Old time flashers used to be directly inside the unit with which you trigger the 4ways. I don't recall that far back but its to the left under the wheel.

If you don't find it there, hunt down the main fuse and it should be near or on it.

 

I know on the newer ones it is an electronic box inside the dash pretty much directly behind the steering column. I is the flasher and controls the daytime running lights.
Had to replace mine and it was about $300 here in Canada.

 

Check kick panel too the left of the clutch. That where most older trucks are located. Or under you dimmer switch panel. Remove two screws and should be there.

 

I have a 05 western star problems with left blinker doesn't work with headlights on turn headlights off left turn signal works

 

But, right turn works?
You have a simple wiring problem. Corroded, bad ground.

 

I have the exact same issue