@turtlevan Yes that's needed, veeery needed, but I think it's so much difficult o impossible to do, and that's why no one did it yet.
@ChOcOsKiZo I have exactly 19 Add-On cars, 10 with Tuning Parts. Also, in a own DLC folder (as I said few comments before), 13 Tuning Parts (for banshee, buffalo, bullet, dominator, elegy2, entityxf, issi2, khamelion, rapidgt, rapidgt2, sultan, vacca) of Add-On cars that are not made replacers by owners (like Ford GT 2005 by Aige or Porsche 918 Spyder by Se7enmoon).
What I don't know, is why 3 cars (EntityXF, PFISTER811 and Issi2), doesn't have tuning working, if all is correctly installed and works when I have 2 Add-On's less installed. Strange things happens to GTA Modding world...