Hi @TTrans,
When you create a custom object you can choose which other objects it would associate with. If you choose to associate your custom object of Cars to Companies, it will show on the right panel as an association similar to associating a deal to the company.
You can create this association when you set up the Custom Object using the API. You will also be able to manage the associations and labels if you have opted in to the flexible associations beta.
Please note, it won’t function exactly like line items on a deal record as you mentioned, but it will function like the deal association on other records.
I hope this is helpful.