Aside from TLM's testimony, there is no proof that they cleaned the car at all - nothing to back up that statement. And she conveniently couldn't remember where that car wash was. I would think if they had cleaned the car that thoroughly, the scrap of fabric wouldn't have even been there. It would have been tossed out or in the vacuum cleaner.
JMO
Ok, but they cleaned the car in Cambridge, and I believe she said it was after that she was cutting the pieces out of the back seat. So maybe it wouldn't have been tossed out.