How would you audit the automobile revenues (say $100 billion) of General Motors?

Let's say that each car and truck has an invoice on the window and in GM's computer system, and that 8 million cars and trucks were sold.

Then,how many invoices out of 8 million should you want to examine (audit)in order to verify the $100 billion of revenues in the financial statements? So, you, as the auditor, would feel "comfortable" to sign the auditor's report (issuing your professional opinion as the auditor) attached to the GM's annual report to shareholders.

Please reply with your first instinct or "gut-feeling" and share your thoughts popped up with the rest of our colleagues in class.