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, http://www.walmart.com/ip/LED-Lighted-Battery-Operated-Flicker-Flame-White-Christmas-Votive-Candle/24565505 and another used this flickering light kit. http://www.modeltrainsoftware.com/flickering-fire-kit.html
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