Ok sorry
Yes Arduino Pins
I will use your library on Github
If I look up a wiring diagram for the LCD I see
GND - VCC - Display contrast - register select - read/write - enable - 8 data pins - anode - cathode
is the back light the same as the contrast you say to connect this to a different pin
If I connected the contrast to a PWM then could I in the sketch dim the display until motion is sensed then turn in up for say 5 min