You can do it that way:
var car_resource = preload("car.tscn")
var car = car_resource.instance()
car.set_pos(custom_position) # use set_translation() if you are in 3D
parent.add_child(car) # parent could be whatever node in the scene that you want the car to be child of
As a suggestion, I would not spawn cars in hardcoded positions. Instead, in every map I would place spawn points (Spatial or Node2D nodes) to know where to position the cars.