There are plenty of external hard drives that only use a USB cable for power, and they're mostly based off a 2.5 inch notebook hard drive. But a "good" external drive has some sort of active (fan) or passive (e.g., heatsink) cooling. If you want a good drive performance-wise, you should look for one that uses an external SATA (eSATA) port, which is much faster than USB.
Also, it's generally not a good idea to leave any hard drive in the car. It can easily get very hot inside, and the heat may damage the drive over the long term.
I'm not saying you're wrong, because I don't know, but I don't think that even in the heat of my car I have much to worry about as far as performance and the life of the hdd... I've been in server rooms that I almost immediately started to sweat in, I also have worked in a steel mill with computers in, near some of the hottest places I've ever been lol...
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