There is always a chassis ground wire in the factory stereo connector, alternatively you can crimp a ring connector on the chassis bonding wire and ground it with a star washer anywhere you can find adequate chassis metal. Look at the factory connector and the adapter, several of the factory connector wires are not in the generic speaker wire adapter, like the antenna preamp power and the factory dash dimmer control wires.
Many of the newer cars may have a radio antenna preamp on the cars with combo XM/Onstar/satellite antennas, the blue acc amp pull on wire will then need to be connected to the proper factory connector wire.
RP; As said before, the new deck needs a star washer to chassis ground, it's not a floating ground unit.
It doesn't matter how the old deck was factory wired and ground bonded, info was for general purposes.