If you need to raise the window, you'll have to:
* Reinstall the tailgate panel: This is the simplest solution. Getting the panel back on will restore access to the motor and allow you to use the power window switch.
* Access the motor directly (advanced): This requires significant mechanical skill and knowledge. You'd have to carefully dismantle the mechanism inside the tailgate to potentially engage the motor manually, a risky process that could lead to further damage. This is *not* recommended unless you have extensive experience with automotive electrical and mechanical systems.
In short, reattaching the tailgate panel is the only practical and safe way to raise the rear window.