Think of the mirrors from left to right as number 0,1,2 (3,4,5 etc if the car has hood mirrors or monitors).
Create an "extension" folder inside the car's folder, and inside create an ext_config.ini (or copy one from any other car's extension folder) and fill it with this:
[REAL_MIRROR_0]
FLIP=1
[REAL_MIRROR_1]
FLIP=1
[REAL_MIRROR_2]
FLIP=1
That example will flip all 3 mirrors. Like I said first, leave whatever mirror you want flipped.
Also, FLIP=2 or FLIP=3 are useful when mirrors are upside down and/or inverted, try those anytime.
(I copied all this from some older GTP post and saved it into a .txt, I'm not that smart)