There is a tail gate relay that sounds like is acting up. Here is a guide and some tailgate wiring diagram to help you do some testing and get the problem fixed.
Please run some tests and get back to us so we can continue helping you.
Cheers, Ken
Images (Click to enlarge)
May 29, 2017 at 1:34 PM
Repair Safety Notice: This information is for general instructional purposes only. Vehicle repair can be dangerous. Verify all information, follow manufacturer service procedures, use proper tools and safety equipment, and consult a qualified repair shop when needed.