Car Wash

So today was nice and sunny, and after all this messy weather we've had lately, I decided to treat my truck to a nice bath. So while I was waiting in the considerably long line for the car wash, I had my window rolled down, smoking a cigarette and enjoying the (comparatively) balmy temperatures. It's an unmanned, automated car wash, so I put my money in, made my selection, and when it was finally my turn, pulled into the wash bay. Can anyone guess what I forgot to do?