Hi there! I am not an expert in this in any way, so I do not want to be at all presumptuous. However, my question back to you would be: where do you want to/ need to see it? GTD is more a methodology than a system. That's what's so great about it. You can adapt any system you like to the natural planning and work flow models.
In your example, you might take the notes about these cars on a notepad, notebook, legal pad or portfolio. When you are done taking the notes (say on Tues), tear off the page and put it in your "In" box. Then, following the model, when you process "In" (sometime in the next 24-36 hours) you will see those notes again and will then make a decision about what they mean to you and what you need to do with them. Your choices will be many based on your decision/ need. You can create a new file for them called "New Car Research." You can place them in a traveling file called "Support Materials." You can transfer the relvant information to your PC and then scan or store the notes digitally. Or you can toss them because you already bought the car and no longer need the notes.
If you like to keep all your notes/ thoughts in one place (like the DRE in Covey), then you can certainly still do that by using a single journal or notebook to keep all your thoughts, notes andmind dumps. The key is to make sure that that notebook, like anything else, winds up in "In" so that you can process the "stuff" in there and make appropriate decisions about what it all means to you and what you need to do about it.
Hope that helps!