1st:
download the ATCC mod posted here a few pages back - it includes a Holden VL Commodore Group A car with fully functional lights and dashboard.
2nd:
open the lights.ini from this car and you will see a number of light definitions
Here is the short list on how the lights.ini works (read the Kunos doc within your AC installation folder for more details).
[HEADER]
VERSION=3
Just the header with version definition of the lights.ini file (over the life of AC different versions evolved, use the latest one - not sure what is the latest? Easy - just open a lights.ini from one of the latest Kunos DLC cars)
[BRAKE_0]
NAME=taillight_a_tail
COLOR=200,10,0
OFF_COLOR=150,10,0
Each light is defined with the TYPE of light (a brake light here) and a running number, starting with 0 for each individual light - make sure you don't have repeating numbers - each TYPE of light has its own running number set (you can have BRAKE_0 and HEADLIGHT_0 but you cannot have BRAKE_0 and another BRAKE_0)
The second line in each light definition defines the 3D object in the car model that will illuminate when the light is on.
Get the object name with the Content Manager Custom Showroom (I wrote a little guide here a few pages back with pictures you may find helpful if you don't know how to). Make sure to use the OBJECT name, not the material name or texture name, etc ;-)
The third line defines the color of the light in R,G,B values.
Some special functions are permitted in the case of this brake light the special function of a color is active if the light is OFF (so you can have a lit brake light functioning as tail light and a brighter brake light when brakes are active).
There are other special functions I won't go into here.
Tip:
Whenever you see an interesting light effect on a certain car, make it a habit of checking out both the lights.ini AND the cars CSP car config file to understand how the effect is achieved ;-)
Tip 2:
Sometimes object names are not self explanatory and a complex lights.ini can get confusing.
I like to comment next to the light number WHAT this definition is for for example:
[LIGHT_6] ;dashboard - speedo needle
This makes navigating the lights.ini easier (the semicolon does comment out anything that follows in this line).
THis is what the dahsboard of the ATTC mod car looks like (vanilla install, no modification of the mod):
View attachment 922002 View attachment 922003