I'm a newcomer to this but enthusiastic. Let's see simple cheap heat engines everywhere! - to demonstrate the principle, to spread the idea that useable energy isn't something you just have to pay power companies for, and to engage the interest of an upcoming next generation of innovators!
(children can have brilliant open minds & imaginations).
Re car cooling, Wouldn't the underside of the car be cooler than the top/inside? A panel of black radiator fins mounted underneath the chassis (like you find on the back of a refrigerator), and a similar one mounted on or right under the car roof where it's hottest, could be your heat sink and source. especially if a moistened layer of, say, felt was laid in contact with the sink, for evaporative cooling... would need replenished though (by regulated drip-feed from a reservoir?). Not sure where to go from there tho...
PS if you develop one successfully, having a heat engine gadget with moving parts visible, operating unattended inside your parked car, could serve as a neat advertising gimmick to passers-by: why not have it labelled 'I run on waste heat to keep my owner's car cool, would you like one too? Contact [number/website]'