Or when you are looking at it, move the single rod connecting the solenoid towards the right and it will pop the top half of the tailgate. Do it a second time, and it will pop the bottom half.
I had to do it on one of mine, over the weekend when I was replacing the upper tailgate and was trying to align in from inside!
The carpet piece can be a pain to take off though.
Either that or remove fuse F15 on the BECM and then feed 12V into it output of the fuse, and then ground the 12V source to the chassis (BECM ground point) and then with the drivers door unlocked you should be able to open the tailgate - as the solenoid is normally permanently powered via F15 - and grounds through the CDL of the drivers door - so putting 12V on the output of F15 will feed the things in that circuit - which happens to include the tailgate :)
'06 4.2 Supercharged Black/Cream "Jess" -- Bought April 2021 (NZ)
'01 4.6 Vogue Black/Lightstone "Snog" -- Bought Jan 2012
'99 4.6 HSE Cobar Blue/Lightstone -- Sold March 2009
'95 4.6 HSE Epsom Green/Tan -- Traded June 2008