I've upped the scale a little on the toy car so that it's not lost in the scene. Placing them closer would have achieved the same effect but the result would have seemed more contrived compositionally.
Applying the tonal qualities of the authentic cars in this scene to the toy car and its occupants, and then adding matching shadows was all that was needed to blend the new car into this environment. The higher levels on the subject's car help to pop them from the scene a little - but are still within the limits of verisimilitude due to the inherent reflectance of the car and the intensity of the sunlight implicit in the scene. Overexposing elements within a composite image can add a touch of 'verite' to the scene as the viewer infers (consciously or subconsciously) that the image is less polished than it could be and has therefore had little work done to it.
The fact that the images are greyscale saves having to match colours.