Vlad wants to be able to play streaming music from his phone to his car stereo. Leo says that if he has a stereo that has Bluetooth support, then he can do it via A2DP. But if he doesn't have Bluetooth support, he can get audio through the lightning connector. If his stereo has USB, Leo isn't convinced that he could route it through Android's microUSB port. It's more likely better to use a cassette adapter that he can plug into his mobile phone's headphone jack.
The chatroom agrees that a cassette adapter is probably the best way to go. Otherwise, he'll need to get a different car stereo, or have an Aux jack installed into the existing player as is shown here.
The problem with an FM transmitter is that in areas with a lot of radio signals, he won't find a frequency to piggy back on.