"Junk in the Trunk" is a term (at least in the U.S.A.) that refers to a woman who has a nice, big butt.
However, I am talking about literal junk in your trunk (or "loot in your boot" for you Brits).
Remember those days that you lived as a JW? Remember sticking lots of literature in the trunk of your car just in case Jehovah "led" you to a "sheeplike" person? You remember! The WTS always stresses that we need to carry literature with us at all times so that we can be prepared to give a witness. Remember how, when you'd open your trunk (or boot, LOL), and there would be Watchtowers and Awakes strewn everywhere? Were you the type who had so much junk in his/her trunk that you were embarassed to open it with worldly (normal) people nearby?
A few years ago, my Pioneer sister-in-law was going through an intersection and she had the green light. A car travelling down the road who had the red light came barrelling through the intersection and plowed into the driver's side of the car. My S-I-L's car spun around and around and the other car didn't stop and fled the scene. According to my S-I-L, she said her magazines in her trunk flew out all over the road. Imagine that, it wasn't cats and dogs raining that day, it was tracts and magazines instead! Kinda like that Saturday Night Live episode when the householder clubs the JW, mistaking him for a killer shark. Remember how the magazines flew in the air?
Ah! Junk in the trunk. The bad ol' days!