futten,
As an example:
Go to Gamedata\Teams\GTC-65|Lotus Elan\601, click on the "car" file and open with notepad. Scroll down to the line that reads - classes="SIM_FULL, SIM_GTC, SIM_GTC65, SIM_Elan, SIM_GTCTC65, SIM_MASH" This is where you define where the car will show up in your game. In this case the Elan shows up anytime you run the GTC-65 class cars. If you type in "SIM_GTC76" in this line, the Elan will be available for play anytime you run the GTC76 cars.
Each car must have one of the classes (SIM_GTC, SIM_GT65, SIM_GTC76, SIM_TC65) in the class line to appear in game.
You can also create new custom classes (I have six different classes with specific cars in each)which we can discuss at another time,