“Cars” are not particularly common. I would suggest to call it “projects” and add a possibility to create projects for each client, which then can be assign to an invoice (Dropdown with all projects of this client). So you could create projects like:
“blue VW Lupo 2012” and “red Seat Ibiza 2003” or just the license plate number.
A report with all invoices of a particular client’s project would be awesome.