Thanks again for the replies. This has been very helpful.

So two things now:

1. It seems like what I really need to know, is what printed parts would actually be able to stand up to being on a vehicle. Up until now I didn't really think that the strength of a 3D printed part would be much of an issue. Some people are printing (and using to some degree) knives and other such tools. How would I be able to figure out if a printed part, would ever be strong enough to handle use on an automobile? I guess that's my real question here. So many of them, I just don't see any reason they wouldn't work perfectly. Then again, someone already said that many of them would work just fine. Perhaps someone could cite a few examples of parts that would seem to be fine, but really wouldn't be strong enough. Obviously I'd never make any attempt to print anything that wasn't already plastic from the factory.

2. Don't some 3D printers have a scanner? I've seen videos where you can put a part in a scanner, and it will make a model of it and send it over to the printer to be printed. I guess I just know so little of 3D printers that I didn't know that they don't generally come with such a scanner. And even then, you have the problem of the original part being broken. You'd have to somehow deal with the broken aspect of the part, so that the printer didn't just print another broken part. Which may be doable, through the use of glue. While the glue wouldn't hold up on the car, it may be enough to make the printer just print the part without the crack in it, for example.