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I drew this up because when I am troubleshooting trailer wiring I have trouble picturing what is going on with the 4-wires in the circuit. There are lots of diagrams showing wiring and colors (yellow, green, brown, white) but I like a visual simulation to show what to expect and I haven't found a good one yet.
Circuit starts in day-time mode where markers, license plate light, and running lights are off. Top, Middle, and bottom switches are set to GND.
Top switch to +12V sims the truck (left side of circuit) turning the right blinker on (imagine you are hovering over the trailer). The truck will alternate between +12V and GND on this wire (Green). Top switch back to GND sims no right blinker.
Bottom switch serves the same purpose for the left blinker (yellow).
Both switches to +12V operates brake lights.
Middle switch to +12V powers marker lights, license-plate light, and running lights through the brown wire. Top and bottom switches should now be set to GND (the truck does this automatically, here for the sim, we must change the voltages to have running lights on). This is how the circuit operates run for night-time mode.
Now the blinkers work inversely from the daytime mode. Top switch to +12V Now turns the light off (+12V - +12V = 0V across the lamp) instead of on (+12V - 0V = 12V). This sims night time right blinker. The truck would toggle between +12V and GND to blink.
Left behaves inversely also.
Brake lights in night mode get tricky. The truck must make both tail lights brighter than they are for running lights. A combination of changing the brown wire to -12V and setting both yellow and green wires to +12V drops +24V across the tail lights making them brighter during braking. Switching in the circuit allows this sim but requires a certain sequence as you will see.
In the truck, the brighter brake lights in both daytime mode and night time mode are actually handled with dual intensity bulbs. Rather than supply +24V across the bulb, the truck supplies +12V with a higher current rating. I may change the circuit to do it that way instead but it's late and I need sleep.
If you know more about trailer wiring and see issues with my guess as to how it works in the circuit please let me know!
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