Not sure why they put dual grounds on your aftermarket tail lights. If they are both indeed grounds you can combine the black and green wires into a butt connector to the factory black and white wire then just match the rest of the aftermarket wires to the factory wires and your good to go.
On all my 12v wiring I use a crimp and solder connector that heat shrinks as well when you heat them up they are great for keeping water out of your connections and available on amazon or from fastenal/granger.
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