In America, police only have to show 'reasonable suspicion' rather than probable cause to search a person and his immediate surroundings for weapons - and if they find something else that's illegal while searching for 'weapons', you're out of luck.
Since the trunk is out of reach of the driver, it can't be part of this 'reasonable suspicion' search - unlike, say, the glove compartment. For the trunk, police need your voluntary consent or probable cause.
In other words, keep your contraband out of reach.
Also, for weapons, you can keep unloaded shotguns and rifles in your trunk. If they're in the passenger compartment, they have to be in locked cases. I have an A4, and it's not really big enough to carry "real" rifle cases like Pelican 1750s, especially with anything else in there. A soft case or bare rifle fits fine when going to the range.
Sound advice, and a good explanation. I prefer not to own (or use) contraband, but even so I would prefer to exercise my rights and ask for a warrant before a truck search.
Since the trunk is out of reach of the driver, it can't be part of this 'reasonable suspicion' search - unlike, say, the glove compartment. For the trunk, police need your voluntary consent or probable cause.
In other words, keep your contraband out of reach.