From
wikipedia:
"Due to the relatively low efficiency, thermoelectric cooling is generally only used in environments where the solid state nature (no moving parts, maintenance-free) outweighs pure efficiency."
Sounds like it's not quite ideal for the automotive environment. Or, more accurately, the fuel efficiency environment. Remember if it's not good for efficiency, it's not gonna be good for fuel efficiency (so long as you're running it off the alternator)
And if you were to run it off it's own battery power, it would probably be more efficient to just remove the alternator and run the a/c off the belt.
Remember, you can always cycle the a/c to squeeze a bit more efficiency out of it. It's a bit more work, but it's free and won't require re-inventing the wheel.