Thanks man much appreciatedHi,
Have a look at this: https://docs.rigsofrods.org/vehicle-creation/fileformat-truck/#flares
And there is also a tutorial on adding lights/flares to a vehicle: https://docs.rigsofrods.org/vehicle-creation/flares/
Let me know if you get stuck on something, I'll try and help
Editorizer, which is mentioned in the tutorial, unfortunately can't open the newer .truck files properly and displays nothing.
You can get the node numbers in game by pressing K while you're in the car, and then CTRL+K to cycle through different views until you see the node numbers.
You can then use the free camera mode (shift+c to toggle) and then WASD keys while holding the ALT key to have a look at the nodes and identify which ones will be suitable to use as reference nodes. Don't worry if they're not in the exact place you want the flare at, you can position it better by changing the offset values.
(I don't know if the reference node and the X and Y nodes you choose have to be connected by a beam, I don't think so. I think ideally they should be pointing at right angles, not sure. Also as long as they are in the correct plane I don't think they need to be going straight - my Y node (28) is more to the right than my reference node but it doesn't matter because it's still in the correct vertical plane)