Welcome to Allpar. The door trim panels will likely have to be removed to get inside the doors. This is awkward with the doors closed and rear seat in the way. You may be able to fold-up and remove the rear seats and remove them from out the rear hatch to get more access to the doors.
It is possible that the previous owner rarely opened the back doors (or rear hatch) and they have stuck shut?
You may want to get a factory 2006 PT service manual with pictures and procedures. The 2001 PT may have some differences, but is shown in chapter 23 (Body) on p. 23-49 here: http://oskin.ru/pub/chrysler-dodge/manuals/Service Manuals/2001_PT_PTCruiser/01PTE.PDF
In figure 42, the link (6) has to be unclipped from the inside release handle once the rear door trim panel is pulled away to get behind it.
Make sure that the latch linkage is connected to the latch. Check for corrosion and try a penetrating oil on the latch? Try working the linkage by hand to see if the latch releases? A good view of the latch is shown in figure 45.