The crosshair uses user input. The game does not compute a position for the crosshair in order to set the X/Y axis'.
The function at 0x60DC50 picks an entity for auto aim, but I'm not sure how easy it would be to get it to work on a car tyre, since it's not an object. You could probably simplify things by creating a collision-less object at the car's tyres and making it targettable with:
|035D: make_object_targettable $1754 1|