I'm really glad you stood up to them. If only more people did this! I would (and have) stood up to them also.
Note that they don't need a search warrant to search your car, unless it is parked at your house. They need only PC, or probable cause - which they clearly didn't have. Again, thanks for standing up to them! Also if your car is impounded for any reason they can also conduct an 'inventory check' of the trunk.