

Solar sail effect is going to be dwarfed by regular atmospheric drag in low earth orbit. At perfect right angles the radiation pressure on the panels is 4.5 micro-Pascals. Meanwhile, in low orbit there’s enough residual atmosphere to generate a dynamic pressure (for drag) of 5 milli-Pascals, give or take (and strongly depending on the space weather).
So atmospheric drag is around 1000 times more than photon pressure. And the drag is big enough to be noticeable over weeks and months, requiring regular boosts to stay in orbit.

Your car radiator is actually using convection to convect heat into the air.
The spacecraft radiators use radiation to dump heat by emitting infrared photons. Photons do not require a medium. This type of radiator works by maximizing the area of hot surface exposed to empty space (which has an effective temperature of 3 K). They have to be pointed into a dark area and away from the sun. There’s no advantage to fins, because you want to maximize area perpendicular to the dark sky.
Both devices are called radiators, but they are different kinds of devices.