I have set up a camera projection of street with buildings. Problem is the photo is way too grainy and my CG car looks to clean. I have added film grain using a downloaded overlay before to CG scenes. But now I need to add only to the CG car and not the photo/camera mapping. How would I do that in Blender? I could easily do that in Fusion, but that would mean renderings different passes from Blender ad adding to the render time too. If I can out it it directly and finished from Blender would be better. Thanks for the help.
you can mix another shader with your grainy texture over your shader.
or by mixing picture together but using window texture coordinates.
Thanks for the reply. I’m not sure if I misunderstood your reply or you misunderstood me.
But What I need is to add the grain to the CG car and not the whole scene. I’m new to Blender compositor but I see that my only input option for the Render Layer node is Scene. I can’t choose the car there, which I had thought would be the way. But I need to add grain to the car and its shadow, done via a shadow catcher.
Now I see an image, which for some reason didn’t show before. Are you suggesting using the shading tab instead of the compositing tab to add noise directly to the car paint? Meaning I will have to add the noise separately to all components, tires, shadow etc?
I haven’t thought about “film grain” in a long time, since now most everything is captured digitally. It would look a little strange to me, unless you were specifically trying for the effect of “an old movie.”
Well, the image to be matched was shot on film.
Besides, most movies, as in Hollywood movies, still add grain to their digitally shot footage. 
Normally, grain is removed from your plate/base using a denoiser and added again after your whole composite image/video. If you only want to add grain to your asset, where your adding your film grain, use fac input with your CG alpha to mask it. If the node don’t have fac input, use Set Alpha node.
Did an example: For testing purposes I’m softening my background to remove part of grain (this is not a good approuch), them I’m regraining again with a digital jitter from Lens Distortion node using a mix to control it.
You will have far better output using a compositing software with Neat Video denoiser and a regrain tool.
Asset: https://skfb.ly/6GGVY
Image: https://unsplash.com/photos/O7cW0a8lVFo