Tailgate won't open

first is diagnosis.
you examine and test it. to see if its electrical or mech. failure.
and once discovered that follow that to logical conclusions.

Hi,empty the boot,check for eney plugs or holes if none,
shut tailgate and get a hose-pipe and shower the car round the top part of the tailgate,quickly wipe off exess wateropen tailgate and see if there has been eney water getting in,if not open tailgate and carfuly so as not to let water go over into the boot turn the hose on to the lamp lenses where the sills are and see if water enter through them if not try be hind the petrol-flap,
my moneys on the tailgate sill.hope this helps

check for proper operation of the tailgate sensor is any where around the tailgate
the tailgate should close all the way if not the message will be there even you change the sensor

yes take off the hatchback door panel and this give you access to the bar that releases the latch

the most likely failure is the tailgate lock actuator, the tailgate will have to be taken apart from inside (very difficult)

I had the same problem. I was able to get the back window open.
I took the the 4 screws off that are on top of the inside of the door (where the tailgate window latch is).
Got a metal coat hanger. straightened it and made a hook at one end.
carefully pushed the plastic part of the tailgate towards the inside
slid the coat hanger straight down (to the level of the tailgate key/handle) to catch the door latch from the inside and pulled up.
that will get the tailgate open.
then you need to take the rest of the screws out of the lower part of the tailgate to get at the latch assembly. Carefully take the plastic off.
(you will need to take the nice black insulation stuff off the metal part of the door carefully so you can replace it and tape it up if necessary after fixing the latch.)
peel the insulation off the metal part of the door, you will see the assembly rod has come off the latch and will need putting back in place and a little sqweeze to stay on.
really bad design I guess, but since I fixed it at 140000, I haven't had any trouble with it at 166000.

remove all the screws you can and gently pull the panel back. you should be able to see down to the latch assembly and reach the release for the latch

There is a small plate on the tailgate door about center and a little to the right about2x3 inches. remove it and with your finger reach in through the insulation and you will feel a lever. that is the manual release. if it will not move the control motor is broken in the locked position. To change that you need to remove the panel on the door and the insulation mat covering it. It will cost about 90.00 bucks from jeep

This is kind of tricky. OK let's start, you have to crawl into the rear hatch area from the inside of the car. You'll have to pull the rear panel off the hatch, a screwdriver between the panel and hatch near the window will work. There might be 2 screws on the panel you could take out. The trick is to pull the panel far away enough to stick your arm down towards the latch and release it by hand. We did this at the dealer all the time.

I don't think you need a wiring diagram.
Jeep had alot of problems with the lock accuator in the tailgate.