Simplified 73-80 Lighting Circuit Diagram (Save For Debugging Issues!)
 

I ended up making a reduced & colored schematic JUST for the lighting circuit. I found this to be a great resource in debugging an issue I had.

https://i.imgur.com/UJazviD.png

My issue:

Turning on the right blinker caused basically every light to flash, including the gauge cluster. Eventually narrowed it down to the passenger taillight dual filament bulb. Somehow the "DK GRN-19" +12V signal was leaking into the BRN-9B wire. BRN-9B is shared between almost all of the lights and is used when you pull out the headlight switch and get the nighttime running lights (parking lights + side markers + license plate + gauge cluster backlight).

Debugging Tips

• * Check all grounds (battery -> frame, alt support, radiator), (eng -> cab), (front lights -> rad support), (gauge cluster -> parking brake assembly), (fuel gauge -> frame), (taillights -> bed, frame)
  * Disconnect turn signal switch & use ohm meter to verify switch. (Right Turn -> Connection between dark green, purple, dark blue), (Left Turn -> Connection between yellow, purple, light blue), (Hazards -> Connection between dark green, yellow, brown, dark blue and light blue)
  * Use a jumper wire to isolate turn signal circuits on cab side turn signal switch plug. I tested purple -> yellow, purple -> dark green, purple -> dark blue, purple -> light blue. Purple -> dark green isolated my issue to the rear turn signal bulb.

Hopefully this information helps debugging any light issues people may have!

Re: Simplified 73-80 Lighting Circuit Diagram (Save For Debugging Issues!)
 

This is awesome, good work!

Re: Simplified 73-80 Lighting Circuit Diagram (Save For Debugging Issues!)
 

Very helpful thank you