Is it the motor, causing my rear passenger power window not to work?
That's hard to say, it could be wiring, switches, froze regulator, or the motor assembly, you need to take off the door panel to gain acces to all of the above for testing.
Tennisshoes is right but it could also be a broken wire in the rubber boot between the door and the body(near the door hinges).
I've also seen them be just plain old stuck where the glass slides up into the door frame. Enough dirt and grime builds up on the top edge of the glass and it can hold the glass up where the electric motor can't pull it down. Try starting your Endever, make sure the window lock button is not on, open the back door, push down on the window switch, make a fist, and hit the window while the switch is down. Now remember your not trying to break the glass but you need to firmly hit it repeatedly towards the top where it slides up into the door frame. If that works when the window is down clean the glass on all 3 edges that fit up into the window frame. Clean all of your windows that way from time to time and you won't have this problem again, unless it is an electrical problem. I hope its an easy fix for you.
Looking for a Used Endeavor in your area?
CarGurus has 525 nationwide Endeavor listings starting at $1,998.
Search Mitsubishi Endeavor Questions
Mitsubishi Endeavor Experts