Bazooka BTA6100 behind penger seat in a sedan?

Yeah you can and it would sound better as opposed to the trunk unless you modify the back panel above your trunk by cutting out some of the metal and what not so your trunk's airspace is the same as your cabin's airspace (hope that makes sense). People use the trunk because they want the cabin space available for pengers and often do not want to put their audio equipment on display for thieves.