Can't unlock liftgate 1996 minivan

On the inside door panel of the rear door you should find a plastic plug about 1" around. Remove the plug, inside you should see a post sticking up. Put the correct size socket on it and turn CW to open the lock manually. Its a good idea to spray WD40 in there and let it soak for 30 minutes to try loosen it up before you try turning it. as its not opening because the lock is rusted and probably seized.

I would bet on the liftgate module (LGM) It controls all that stuff back there,wiper,defrost,gate ajar light,gate lock and unlock ect. Might try wiggleing the wiring boot where the wires go from the body to the gate,if it changes anything,bet on a broken wire inside that boot.
If not the LGM is my first thought.

It can be faulty switch or its issue with lift gate latch.Its got stuck up hard or cracked inside.It has to be inspected.-----------
There are few help links with proper solutions for these problem.I suggest you to refer the links.
Click the link below:----
2007 Mercury Mountaineer: Liftgate will not Unlock or Open? http://whoisbyme.blogspot.in/2012/06/2007-mercury-mountaineer-liftgate-will.html
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Adjust Hydraulic Lifters? http://schematicsdiagram.blogspot.in/2012/04/adjust-hydraulic-lifters.html
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How to open rear lift gate? http://repairhelpcenter.blogspot.com/2011/12/how-to-open-rear-lift-gate.html

--------- Power Liftgate Shuts Down http://whoisbyme.blogspot.in/2012/07/power-liftgate-shuts-down.html
------------ Power Lift Gate Will Not Remain Open http://whoisbyme.blogspot.in/2012/07/power-lift-gate-will-not-remain-open.html
----------- Power Lift-gate will not Operate http://whoisbyme.blogspot.in/2012/07/power-lift-gate-will-not-operate.html
------------- Liftgate will Not Stay Up Open? http://whoisbyme.blogspot.in/2012/07/liftgate-will-not-stay-up-open.html

---------------- Rear Door will not open http://whoisbyme.blogspot.in/2012/07/rear-door-will-not-open.html
------------ 2000 Toyota Corolla: Rear Door wont open from outside or inside? http://whatisbyme.blogspot.in/2012/05/2000-toyota-corolla-rear-door-wont-open.html

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These details will help.
Thanks.

the shock are bad

spray the latch down with starting fluid to remove grease then spray it with silicone to lubricate as grease will bid up lock lever worked for me

do you have a second fob try it battery may be low may need reprogramed or replaced check with your dealer

can you hear the latch release when you try to open? if no clicking noise is heard then most likely a bad latch assembly. also check the rear door control module to see if smells burnt, if it does it won't send open signal. the module is located behind the plastic shroud on the left of the rear hatch.

need to oil the latches they are starting to freeze up use some wd-40 on them open and close the gate several times spray again repeat until it opens freely

Remove the trim from inside the vehicle and release the latch rod manually. Once it is open you can further diagnose the problem. It may just need to be lubed from the inside.

Replace the lock on the rear door. If a used lock is used it has to have the powered door. I am a car dealer and have replaced about 8-9 of them. I sell a lot of Town & Country vans. The 1st one I paid a Chrysler dealer to fix. About $200. After that I fix myself. About $70- $125 from a Chrysler dealer new with discount. The lock is not sending the correct code to the computer and the door is confused on what to do. Only 3 torx bolts to remove and the door panel 2 screws in handle then snap off.
On one I had that did not fix the problem and it was a relay sensor about $40. on the working arm / motor of the door and not the lock. Look at car-part.com for used parts. Thanks, Roy