Great idea. I know a few people who did something similar with a space heater in the winter.

Anything that would be large enough to provide adequate active cooling would require hard wiring. Depending on the enviromental conditions (namely humidity), you could likely use something like an evaporative cooler, which takes not much power.

They had those in the 50s my Friend .The unit hung on the outside window & a compartment filled with ice & a fan blew cold air inside the car . The fan ran via a cigar lighter plug . Excess water drained outside.No damage to the Ozone .

Unless there is a signifigant breakthrough in small electric air conditioners, it's probably more cost-effective to trade the non-AC car in for one with AC already installed. 12V air conditioners do exist but they would have to be extensively modified; a bit expensive for a one-off installation; for a consumer product that would have a very limited market, the R&D would be fairly expensive, which would make the product kinda pricey.

You can get a DC powered thermocooler (peltier cooler) that runs on 14volts to remove about 82 watts with a eighteen-amp draw. That's about the most I'd consider taking out of a cig lighter.

http://www.melcor.com/maa280sp.html

82 watts won't cool the air much in a car, but it might cool the person. I know that these thermocoolers are the technology behind the astronauts' and pilots' 'cool-suits'--they use peltier coolers to cool a liquid and pump it around the person's clothes with a network of tubes. A related take-off of this might be to try designing this into a bucket-seat and see if a cool bucket seat can make a person confortable enough?