You really can't talk about the DUI portion that has to do with alcohol. You can always ask general questions about driving while impaired and asking questions about being impaired or the marijuan aspects of your case.

Police are allowed to search a vehicle that is being towed following arrest. They often times use this excuse to get around the warrant requirement. It basically allows them to search and inventory a vehicle prior to towing to prevent the owner from accusing the police of stealing from them. Anything found, if the cops dot their I's and cross their T's and follow procedure basic procedure established for towing following arrest, is fair game.

If you were arrested and booked, chances are better that you leave the pipe in the car. Introducing drugs into a jail or prison is a felony while possession of paraphernalia on your person or car is usually a misdeamnor. I'm guessing bring the pipe into jail with you would probably be a felony. They should search you when booking you but if they don't find the pipe and you bring it in that would seem to be a more serious offense than possessing it outside the jail.