You may try to do it like this:
(Warning: This is just my mumbling about stuff)
You know size of vehicle box(xyz), position of car is center of box (o).
You know angle of car box in degrees.
You know distance(d) between player and center of car box.
You know angle of player camera so you can calculate exact point(xyz) where player aiming,to be accurate you may use Map Andreas Plugin for z of ground at xy of POV .
Now you have point in 3D space(xyz).This place where you bullet(hit) while pass.
BOX OF CAR: Have center point at car coordinate and half + and - size of Vehicle_Model_Info for each X,Y,Z plane in appropriate angle of car.
Now you must test is this point belong to any of BOX OF CAR(plane).
And if you have a hit else you not.
This is actuate.
http://tutorial.math.lamar.edu/Class.../3DCoords.aspx