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Radio wiring is pretty standard for volvo
black: ground
green: constant +
blue: switched +
red:power antenna, or amp remote on
speaker wires:
left +: blue/white or yellow white
right +: blue/white(smaller blue stripe the left) or yellow/gray
left-: white
right-: grey
The dim wire should plug into a yellow wire in the dash. It will be a loose wire, separate from the radio wire bundle
There is also usually a brown "rheo" wire that plugs into a separate brown wire in the dash for the panel lamps in the radio.
The stock radios have no rear speaker outputs.
They may have a 6 pin din plug to plug in a Volvo power amp for front and rear
The din plug can be adapted to other amps with difficulty (I have the pin diagram somewhere if you want it)

The only big mistake most people make is to assume red is the positive lead.