With the harness not connected to the deck, you're probably gonna have to re-install it briefly. If you can easily get ahold of a harness, just hook up the remote, constant and ground to any 12V power source, and that would do the trick.
If you want to use the harness in the vehicle- Instead, just use the keys that came with the new deck to remove it, and connected to the harness adapter will be the stock harness. It's a lot easier than removing the trim piece.
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