100$ on the light switch...Or the harness in between... I will explane in detail as wiring confuses the snot out of some.. If you understand it just bear with me on this as I will put it into as simple of terms as possible...
The path to follow is this... Battery (or start relay) to the light switch... light switch to the turn switch... Turn switch to the lights...
In detail...
Battery to #10 wire... # 10 feeds most of the truck (ign switch, circuit breakers, etc) and is always hot... so... if you have ign you know this is a good circuit. (If not, its the harness)
#10 wire feeds the # 15 wire... which feeds the light switch for all of the lighting on the truck... which again is always hot... #15 is Pin F on the light switch connector plug (back of the light switch and wiring harness plug)... if you have any working lights (head, tail, etc) # 15 circuit is good... (if not its the harness)
If you have SOME working lights... Erase the above from your brain and move forward...
In the light switch you have the Service Drive position... (plus others but forget those)... Were looking at Pin F (#15 wire from above) and Pin J (#460/461) these two make contact to power the "turn" circuit...
Since we have headlights we know the switch is powered (or not powered)... you should have voltage output on 460/461 Pin J.... and pins B,M,A,K,C but they don't matter as we are looking for turn...
To test the light switch... remove it...
In the Service Drive position connect your meter to Pin F which will represent the battery feed on #15... and test for continuity to Pin J which will represent 460/461 output for the stop/turn circuit... If you have a good connection the light switch is working to feed the turn switch.
If you are missing anything between these you have a bad light switch... Repost and I will move forward if you need the help...