I just realised, that with my new custom light mod, that I could've made it add a new taillight design selectable from the parts menu instead of it overwriting the normal lights. How do I do this?
Why not take a look at a mod such as this one: http://www.beamng.com/resources/torque-thrust-barstow.102/
You could do it with the new skin system. Take a look here. http://www.beamng.com/threads/introduction-to-skin-system.16931/#post-265581
It appears to me that the game does not accomidate what you want to do very well right now. At first I was going to advise you to look at the "Front Fascia" and "Rear Panel" since that's what the lights map onto... but the .dds files are called in the materials.cs & the glowmap and I don't see a good way to swap that stuff out by itself yet. I don't think doing a whole-vehicle skin for euro lights is exactly what gigawert had in mind!
Maybe I'm being thick, but even with the jbeam (like on the pickup) I don't see how you can achieve what you need to do. :-(
It may just be that what's needed here is (way) beyond my knowledge. Everything you want to work with *does* have it's own part already - either the Front Fascia, Rear Panel, or whatever. You've got a .dds file already - that's done. The .dds files seem to map to a name in the materials.cs file for the vehicle, for example: mapTo = "hatch_lights_on_intense"; we could create a folder for your mod and put the .dds file in it along with a materials.cs file which maps that to... what? I think mapping to hatch_lights_on would be a mistake since we've already got something by that name (the stock stuff). I guess this should be arbitrary and unique. Then I guess your textures would map to the replacement fascia/panel/wahtever in a .dae file. I'm really out of my league on this though.
Copy the original lights to a new jbeam file, then do what you did to overwrite the original one. Give it a new name and a new "codename" and it will be a new part in the list.