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Your powersupply should have come with a power cable to hook up a standard 3.5 drive:
You could also use a small inverter to go from 12v to 110. This will allow you to use any external HD case on the market, many with built in cooling fans). You do not need a large inverter and 100w should do the trick just fine. You can hook the inverter to a line (like your stereo) that turns on when the key is turned on.