Just bought a 2006 Forester, it had the following condition when I bought it:

 

The rear passenger door Drivers side will not open with the interior or exterior handle

 

Access to remove door panel and get to linkage is limited if possible at all

It could be that the lock is damaged or the child lock is on and the exterior door handle is broken.

Remove the right side door panel and figure a way to stick a coat hanger in through the window slot to pop the door latch. Practice it a few times then try it on the broken door.