port placement

A trunk, a room, a cab of a truck...all have a common airspace.  When the port produces sound, which it will at various degrees throughout most of the freq range, that sound needs to blend with the sound coming directly from the driver itself.  It's the same way, in a manner of speaking, that the sound from a sealed box blends with the sound of the driver.  The total sound is a blend of driver and box sounds.  If one were to purposely port the vented box through the rear parcel shelf while the driver fired into the trunk cavity, he would hear the sound from the port at a different time and db level than it should be.  But if both driver and port fired into the trunk cavity, he would hear the result of the mixture of sound...even if the port were open to the rear of the box.

The way to open up the trunk to the cabin with a ported box in the trunk is the same way  you'd do it with a sealed box.  Fashion a hole, or large opening, through the parcel shelf or seat back.

A port can extend from the box, yes.  There are often ways that one could have avoided having such a long port, though.  Usually, it's using too small of an airspace for the driver which resulted in an overly long port, or it's porting a driver that would probably have been better off used in a sealed box.  In car audio installation, customization to fit the vehicle is a primary concern.  One may have just enough space in the car to design a box but with the added displacement of the port it becomes too huge for the space.  In a case like that he could extend the port from the box and fit the contours of the car better without compromising the amount of airspace he wants in the box itself.  It's important to leave an amount of free airspace on both ends of the physical port structure, shown below in red.  Those spaces are actually part of the acoustical port properties (don't end your port opening against a wall).