Reach in there, find the two nuts that mount the window to the guide, take
them off with a wrench, and carefully slide the window out. Have someone
help you.
Spdloader

there is a switch in the latch to prevent the window from moving when the
tailgate is down, it can be bypassed by closing the latch manually.
the window motor should have wires going to a connector that is accessible
without raising the window, unplug the connector and put power to the motor
directly to test the motor

The window requires the tailgate to be closed for it to work properly, but
working on the inards of the gate is difficult when it is closed and the
window is up.

I used to activate the latch mechanism on the open gate, then use the key to
raise the glass while holding it up. This allows one to work on the stuff
inside the door. A serious problem with this is that the window is very
heavy, and you need to devise a means of supporting the weight or risk
having the window break.

If the window does not know that the tail gate is closed, it won't go up or
down. If it went down as the last thing it did, then it won't go up again
until the Door Closed circuit is happy.