Your car is basically a mobile dining room, which means crumbs and wrappers are the number one party guests for bad smells. Check under the seats. I once found a half-eaten apple that had evolved into its own ecosystem under the passenger seat. It smelled like a science experiment gone wrong.
Get out everything—water bottles, old receipts, that gym sock you swore you lost last June. Everything. A quick vacuum doesn’t cut it here; you need to act like you’re moving out of the car forever. Trust me, your nose will thank you for the eviction notice.