Happy to [try to] help.
Honestly, I have no idea where the sensor is. Without having previously worked on the G500, all I can say is that's something another tech would have to answer for you - preferably a Lenovo tech. I don't think the cable can be connected improperly very easily; from what I've seen on most systems, the connection at the monitor can go either way, while the connection at the board either can also, or it has a notch in the receptacle to prevent improper seating.
Do you know which type of cable you used (LVDS & pin count, or eDP, etc.)?