The blue wire on the deck is a signal sent FROM the deck to a device like a power antenna or amp. Its not any kind of data signal, its just a low voltage "wake up call".
if you have no amp and no power antenna (like a manual antenna or fixed one) then you dont need to use that wire.
if you have an amp, you wouldnt want to splice that wire to your factory harness/adapter, youd want to splice it into the single wire running to the amp for on/off
if you have an OEM/factory power antenna in your car, connect it to the wire in your factory harness/adapter
if you have BOTH power antenna and a
