Remove the access panel from the inside. Use two jumper wires and connect 12V directly to the motor by separating the elec. connector just ahead of the motor. Jumper wires with alligator clips help a lot. If this doesn't get the motor to do anything, you have to unbolt the glass from the regulator and let it down by hand far enough to get the tailgate open. After this, its a matter of troubleshooting the wiring to the tailgate (LH side), the dash switch vs. the key switch (is juice getting thru) and the safety switch inside the tailgate. I just use a splice connector to bypass this little jewel. This lets you actuate the window with the tailgate down, so be careful to support glass when doing this. The ability to actuate the glass with the gate down is very handy when troubleshooting the rest of the stuff in the gate.
Lastly, there is a rod off the windor regulator that won't let the tailgate open unless the window is all the way down. If this is messed up, you will have to reach in from the back of the gate and push this lever while operating the latch with the other. I had my gate apart about 4-6 times before I got everything working again. Good luck.