Greasing electrical components is not a very good idea; it usually damages them.
The switch that signals teh GEM that the liftgate is ajar is located in the liftgate latch. The switch is a common failure item, and all Ford dealers will have it in stock; it costs around $10.
To replace the switch, you will need to remove the interior panel from the liftgate for access to the latch mechanism. If you follow the electrical wires to where they enter the latch assembly, they will be plugged into the switch. The switch is removed (you usually have to work by feel) by depressing it slightly and rotating it, then pulling upward.
I hope this is helpful; if so, an accept would be most appreciated! thanks!