Hi Thorsten,
I think using a square grid would be a big waste of space since the cars themselves are already quite long in the vertical direction and the lights just add to that direction. By moving them to the centre of their rectangular slots, I managed to fit the light map without any issues. I used the blue channel for the front lights and the red channels for the back lights, since they will need to be tinted with two different colours in the sim (yellowish for front light and reddish for back lights). I tried using the green channel for the heightmap, but since I have to create that manually and it is a lot of work I did it just for one single car so that we can test how it looks like. If it looks good then I would gladly do the same for all other cars. And then we still have the alpha channel to be used in the future.
Here is what I got:
What do you think? Let me know if this works better.