I have a 2015 vnl670, I have tried this not getting the same result. In fact hooking it up this way the turn signal bulb dose not flash at all nor with the parking light on the side don't light up. Put ground back to original position, turn off key, turn key back on is the only way to get it to work again. Don't understand what's going on. I have l.e.d. bulbs.
It would take some additional circuitry to make it work. If you were to sketch out the circuit and look at which way current flows when the bulb is lit, it's in opposite directions depending on whether the marker lights are on or off. With an LED, it will only light up with current going one way. It also relies on current flowing through the marker light circuit to ground when the headlights are off, and if you're all LED there, we run into the same problem. While it definitely improves turn signal visibility, it's a lot more work on a 3rd gen truck. It would have been a lot better if that side marker had a third wire with the logic on its circuit board to flash from the factory.
Why would you want the 2 lights to alternate when headlights are on? There's a simpler way to do this to make the marker light (or any additional lighting you might install) that keeps them Synchronized. I've installed many Custom LEDs (my own creations) on my rig and have never had any timing issues nor any Low Power Dimming issues. Having the lights Alternate is actually a negative effect on the relay and can burn it up quicker.
When the headlights are on, the side marker light is already on, as required. To flash it requires it to go off when the signal is activated, otherwise you'd have to separately interrupt the marker light circuit. There's no relay involved in this setup--only a floating ground.
That's for some people, in 3 years never had any major issues and my truck has 1520000, same main factory installed components on the engine, turbo, egr system etc.
@trucking shenanigans or when you stop at the fuel station your friends 680 with the same engine and transmission isx and 13 speed and same run you put 42 gallons less go vnl
