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Hello my name is Angel
Sound to me like your Liftgate struts are not working any more.
You will have to replace them, they are not serviceable.
but very easy to change, you will need a helper to keep the liftgate open while you replace the struts.
all you need is a small flat head screwdriver. once the liftgate is open, you will find two long block cylinders with shinny long cylinder coming out of the black one( one on each side of the liftgate). At each end you will find a gray clip with two indents above and below the clip.
use the small screw driver to pry out the clip, one removed, that point can be removed by pulling it away from the body.
do the same for the top. and the other side.
to install (make sure in same orientation as when removed. then just push the end piece and it will clip on with a snap.
and you're all done.
If that helps you,
Please rate my information.