My radio already had a black harness being used (the one with the 2 wires in it)

In the instructions, they ask you to retrofit that one instead.

To my dismay, after installing it, I had the same problem as you.

BUT THEN!, I realized the problem.

In the diagram they provide you with, the first 2 harness slots (on the right) are blacked out. This is the location of the existing wires.

When i originally installed it, I thought the "X" was the existing wires so my connectors were literally 1 space to the right, too far.

DUH DUH DUH!...

Once i switched them over to the left 1 space, it WORKED!