Aftermarket NAV in my new H3

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I just installed a the Z1, but used the Peripheral adapters (one for the chime). Best suggestion I would give is write Metra and ask them, it probably requires a different adapter.
As for Onstar, you adjust the sound via the sound knob/rocker on the radio, same as you would with any radio. Even with steering wheel controls, you could adjust the sound directly on the radio head unit.