Best thing to do is to run a brand spanking new +12v from the battery, or optionally from the ignition barrell depending on which way inclined you are.
Then all you'll need is the ACC wire, which by memory is a blue with a white stripe, or something similar. Check it with a multimeter or test light to see if it comes on when the ignition is turned to ACC.
Ground it to a clean point on the chasis and since it has no stereo currently run your new speaker wire. Unless you want to use the speaker mounted up under the dash