Additionally to extras, MISS-ELS has support for miscs as light sources. This enables the author of a vehicle to use up to 38 (12 extras + 26 miscs) light sources!
When you ship your misc enabled vehicle, make sure to include the MISS-ELS modkit in a (separate) carcols.meta file. Add the kit to your own file or include this file as carcols.xml. Read more about streaming these files here.
Then, your vehicle should reference to miss_els_modkit in it's carvariations.meta entry. See the example file.
Now, you are free to use any of the 26 miscs on your vehicle model (YFT).
As a vehicle author, you simply add misc_x to your vehicle, just like you would do with an extra. You can use misc_a till misc_z.