I'm not sure the Peltier cooler is going to help. For one, I think the electrical demand would be too high given the number of coolers needed to cool the large interior space of the smart. (Yes, in terms of thermoelectric cooling, that's a large space!) In a typical beverage cooler application, 60 watts of power is needed to cool an interior volume of no more than 1.5 cubic feet approximately 35 degrees. And that's without the solar heat gain fighting against you.
Further, the time required to bring the ambient interior temperature to a comfortable level would mean that all but the longest car trips would be sustained in uncomfortable heat while you wait for the car too finally cool down to a reasonable level just as you arrive at your destination (or, more likely, it never cools enough during the whole trip).
Additionally, there's the issue with humidity. These devices are much more effective the less humidity there is. If your environment is particularly humid, your results will be unsatisfactory.
Finally, this would have to be installed in such a manner as to draw the hot air out of the vehicle, as opposed to blow cool air into the vehicle. In a typical application (say a drink cooler), there is a fan used to draw the warm are out of the enclosed interior.
I would think your only two viable options are to buy an auto evaporative cooler (again, only really effective if your humidity isn't high) or find someone willing to do the digging to install the A/C unit. An auto evaporative cooler will cost a minimum of $300. No installation needed, though.