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What's the max amp draw of the compressor? Being an electrical guy, and knowing how tempermental computers are, I always recommend installing any additional power source that draws 10 amps to run through a relay.
That way you can tap into almost any hot wire under the dash (other than computers signal circuits), run this wire to one end of the switch, from the other end run the wire out of the dash.
Hook this wire to the switch side of the relay, find a ground, and on the coil side, tap into a direct B+ wire. From the other side of the coil connect to the compressor. In addition, it is very important that the B+ wire is fused as close as possible to the source of the power to prevent any wiring damage, and fires.