Here we go:
-black: yes, the ground wire
-red/black: ignition wire (0V with key OFF, +12V with key ON)
-8 wires for the speakers that you have identified
-yellow/red: to power antenna
There is no brown wire listed in the wiring schematic, and I'm no where near my car to check, sorry. But I don't think it will be relevant. Oh, unless as below, it was added by a previous owner as a new main power wire.
Note that the original radio relied on only an ignition/switched power wire (0V with key OFF, +12V with key ON), and the ground wire. Modern radios have a main constant power wire in addition to the ignition wire and ground wire; the 'constant' wire (ie: always live) is the main power supply and provides memory backup for your settings (radio stations, etc).
To hook up the Sony, I suggest:
-adding a new fused power wire directly from the battery or fuse box, to function as the 'constant' main power wire; should be thick enough to provide 10A of current (or matched to the fuse on the back of the Sony), and connect to the Sony yellow wire (as above, perhaps the brown wire was added to be this wire, so check with a multimeter and trace where it originates from)
-connect the Alfa red/black ignition wire to the Sony red ignition wire
-run a NEW ground wire that is the same thickness as your new main power wire, and ground it directly to the chassis, removing any paint to bare metal; the original Alfa ground wire is too thin for a modern high power headunit
-blue Sony wire can connect to the Alfa yellow/red antenna wire (if I'm not mistaken, Sony - like Pioneer - have only 1 blue wire which is used to connect to an antenna and remotely trigger any amplifier; other brands have a separate antenna and amplifier remote wire; with the Sony, the antenna will always extend/go up regardless of listening to the radio or CD/auxiliary; whereas headunits with a separate antenna wire will only activate the antenna when you select the radio to listen to)