adding onto existing tailgate contact

I am trying to add another magnetic contact to a tonneau cover by splicing into a existing tailgate contact

on a DEI alarm system, I bought a no/nc contact tried both sides cant get either to work?

go back to basics if i have one wire coming from the alarm control to the original tailgate switch,

tailgate switch has 2 wires on it one to ground, one to the wire coming from the alarm control

where do i slice into to add other contact

i worked for a alarm company for ten years (residential / commercial) i know nothing about car alarms

so i would go from single wire coming back to one switch out to other then out of second to ground