Your smart phone FM tuner will not work properly without an external antenna mounted on the outside of the car. The wavelength of the radio signal at FM frequencies is about 3meters. The windows into a car (with the possible exception of the windshield) are less than 1/4 wavelength across, and they greatly attenuate the FM signal. The car body makes a very effective Faraday cage. For a radio wave to propagate through an opening in metal, the opening must be > ~1/2 wavelength.
I know this from trying to transmit/receive from inside a car or aircraft using a 147MHz VHF walky-talky (adjacent to the FM band, wavelength is 2m) .
Receiving FM broadcasts inside my all-metal aircraft hangar doesn't work well either, at least not until I open the hangar door, which is about 7 wavelengths across at 100MHz