Welcome to the forum!
I do not believe there are any official written instructions, although it has been suggested to the people that have modified their cars. One man used the flame from a votive candle,
and another used this flickering light kit.
Flickering Fire - The perfect way to light your fire. For Miniature hobbies, you can create a realistic flickering fire using these LEDs.
Wiring either of these products in is not that difficult if you do not wish to preserve the "disappearing" function. These men have preserved that function, increasing the complexity of the installation.
Larry