depends how your blue trigger wire works. If you are just using your headunit's remote antenna lead to turn on your 420 with the blue wire, then you'd be putting an unnecessary strain on that circuit on your deck. If you have a relay between your headunit and your amps and 420 to turn them on, then adding the bt unit to that circuit is likely OK.
All that said, if you typically turn your battery switch to off at the end of the day, I'd just go to the red wire on the 420. That way, once you pair your phone with the bt unit, every time you power up the boat your phone will already be paired by the time you fire up the stereo. No lag waiting for the phone to pair with BT. And it's not like the power draw from the BT unit is significant at all either.
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