Each car and each livery has a .car file
All cars are located in Gamedata/Teams/folder of the car or car group/
In the chosen car/skin folder .... you have a .car file among other files ( mostly texture files )
In some mods ... .car files can be gathered in the car folder or in a subfolder for its team.
In order to know which is the car that you wish to modify, you have to remember this
A .car file is mostly named with 5 numbers at the beginning: the 2 first ones are the year and the 3 other ones are the car number ( sometimes even 2 letters are added in order to create a more precise car in case of different liveries for different tracks in the same season ).
I'll add an example of an original car within a few minutes.
EDIT:
If you search for a Mosler 2003 N°104 ....
The location of the file is: Gamedata/Teams/24H (where it mostly appeared in 2003)/Mosler Teams/Perspective Racing.
The car file is: 03104_G2_Mosler_MT900R.CAR
The file opens with notepad, can be modified and saved ( without changing file name of course ).
NB If you don't find how to do ... choose a car and I might do that for you.
But always better to do it by yourself in order to learn how to do for the next times you wish to modify something.