quick question about headunit wiring

[Mason]

so i bought a new deck and i just finished assembling the mount I bought now it's time for the wiring. i bought a wire harness adapter thing for 92' up gm cars and what I think i have to do is connect the matching wires to the deck wires and plug it in to the existing harness in the car (correct me if im wrong please)

anyways on to my question. the wires for the deck have a black wire and so does the harness adapter thing... which one do I ground or do i have to ground both