In my old '95 Pathfinder I ended up using self-tapping screws to attach the amps to the rear seat backs. Worked well enough, but for the Pajero you'll have to route extra cable for the sliding seat if you use it.

Other ideas:
-attach to rear door
-hide under/inside 3rd row seat
-use a 5 channel on a sub box to run everything. Make some quick disconnect system to remove the box and amp and revert to deck power only.
-make an overhead tray in the rear to hold audio equipment out of sight.

There are only so many easy options.

1991 Mitsubishi Pajero XP TD LWB family hauler.