[bcj]

If the plug is directional, The wires can be assembled in the wrong order. It's less likely though.

You can test the assembly with temporary wires directly from the battery terminals, taking note of positive and negative polarity.

Most crimp on wire ends used in plug assemblies have a spring tab to hold them in.
A micro flat screwdriver or other thin tool can be used in the plug end to compress the tab and pull the wire and crimp end out the back of the plug.
If you need to re-arrange the wires.

Plug and play should work properly, but where do you use the plug to tap into the middle of existing wire run?
The way it was explained, it's being spliced in.