Since this is a simple connection involving GND connections, I would use any type of crimp on insulation displacement connector.
https://www.zoro.com/3m-displacement-connector-18-14-awg-pk50-801/i/G1141777/?gclid=CjwKEAiAlZDFBRCKncm67qihiHwSJABtoNIgaiYMGVKuZd2R7cF8mAyDKfBdmAhcprkU-U5VZADqvxoC7PLw_wcB
Crimp at the connector on the liftgate switch ( pin 1 and 2) Connect a momentary contact pushbutton switch.
I may install a switch also since it is easy and makes sense ( as long as the doors are unlocked)
The plastic panel on the liftgate snaps on in about a dozen places. Buy a tool to pop the pins so you don't break any (any variant of this tool should work, the plastic ones are also useful)
https://www.jbtoolsales.com/sg-tool-aid-87810-upholstery-clip-removal-tool/?gclid=CjwKEAiAlZDFBRCKncm67qihiHwSJABtoNIgr3ls_h_Gm3KlHD_ae_Zfm96iI97P22CmXL7k4fVVBhoCkXXw_wcB