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You might be better off running at least one new wire anyway. A few months ago I ran a heavy wire directly from the terminals near the battery to the yellow wire, and it improved the sound and the volume I was able to get out of my Alpine head unit. The Volvo wires are all on the thin side.

The red wire can be hooked to a wire that gets turned on with the key. And you need a good ground.

Consider running heavier speaker wires as well if you have good speakers, which means that if you put aftermarket stereo in you can just bypass most of the stock harness. Even cheap 12/2 or 14/2 will probably sound better than the stock wires.

While I was doing this I also added a switch to the center console that lets me play the stereo with the key off, good for when she's shopping for clothes. I ran an aways-hot wire from the fusebox to the switch, then ran a wire from the switch to the red wire. I put diodes in both sides to prevent activating something else, which might drain the battery faster. And I used a lighted switch (junkyard, coupla $$) to remind me to turn it off.