[Silveredwings]

Unlike a photovoltaic panel, your radio frequency receiver is a resonant circuit excited by sympathetic oscillations originating at the transmitter. Though such a circuit seems passive, it actually creates a load on the transmitter (in the same way the transformer secondary winding loads the primary).

This load is infinitesimally small but is measurable with a grid-dip meter. Of course, the amount of current is equally small (on the order of micro amps, or even pico amps). That's why a typical superheterodyne radio must have an RF amplifier followed by an IF amplifier to boost the signal strength enough to pass through a demodulator without being lost altogether. You'd die of old age long before you charged a flashlight, let alone a car battery. There's probably more energy available from parking under power lines, but it still wouldn't get you very much.

It's not technically impossible, it's just that the power is 10 to 12 orders of magnitude too small to be practical.