The locker sensor (plunger) needs to be behind the plate on the side gear, ie: the plunger needs to be installed before the diff, and when the diff is installed they need to make sure the plunger is hooked over the plate. If you don't one of two things will happen, a/ it will lock correctly but the dash light will just flash as it doesn't know it's locked, or b/ the plunger may exert enough pressure on the side gear plate to engage the locker and it will be locked all the time.