I believe in the original design, only the power to the radio memory or radio clock is supposed to be truly permanent.
My Fusion stereo has one (+) that is for "battery", i.e., a permanent connection. This power input powers the clock inside the Fusion only, and this power input is incapable of powering up the Fusion itself. Power is provided by the "permanent" DC-DC converter.
There is another (+) into the Fusion which is for "switched power", i.e., a switchable power connection. This power input powers the balance of the stereo functions. This power is supplied by a different DC-DC converter than the one that supplies the permanent connection.
I would not connect anything else to the permanent power supply.
SM2K N. 350 (2002)
At anchor, Neapolis (East Macedonia)