Check the gasket around the rear wiper shaft where it goes through the liftgate window. Look for cracks in the gasket.That's where my issue was. Body shop tore the gasket that the shaft goes through and that allowed water (rain, carwash, sprinkler, whatever) to drip down inside the liftgate and get on the switch. Once the dealer saw that the gasket was torn, they replaced the gasket and the switch and (I think) installed a plastic cover over the switch and wiring that is supposed to keep any water that gets inside from getting on the switch and wires inside. Once the gasket and new switch were installed it hasn't failed again.
Could also be corrosion in the connectors inside. If you can remove the inside cover on the liftgate you could unplug and replug the connectors several times to see if that helps.
Worst case the actuator could be going bad? Hard to easily diagnose that. Especially if it's an intermittent problem.
Good luck and let us know what fixes your '16.