Empty chamber, remove magazine and put it in the trunk. If you want to be super safe put it in a locked box in the trunk.
If a cop pulls you over for whatever reason, don't tell him about the gun in the trunk.
If he asks if you have any weapons in the car kindly reply that you don't want to discuss the content of your car with him.
NEVER consent to a search, let him get a warrant (or try). They may try to intimidate you into consenting, but hold your ground.
If he searches w/o consent, say nothing to him, call a lawyer.