A thank you note is never a bad idea, but the guys at the car dealership probably wouldn't care either way. And don't wait a week to send it, they've probably already made their decision by then.
You probably didn't want to work there anyway. I guarantee they hired somebody with bigger breasts than you.
BubbaHoTep is totally right. Unfortunately, it's a numbers game. As soul-crushing as it may sound, you need to apply to everything under the sun, including jobs you probably wouldn't take even if they offered it to you. The hardest part is only hearing back on one or two out of fifty applications. But try not to get discouraged.
By the way, do you think you interview well?