Google: car radio modulator
The modulator, in this case, generates an FM frequency, a carrier signal, just like a radio station does.
Best one would be the "hard wired" type, they have an antenna in/out, you unplug the antenna from the radio then plug it into the modulator and the modulator's antenna wire plugs into the radio.
But they do have simpler "broadcast" type modulators, easier to install, but subject to interference just like regular radio station reception.
Both types require power and you can tune them to an unused frequency in your area.
Then you have the "input" connection, these vary as well, standard is male headphone jack on a cable that just plugs into any portable media player.
But you can get them with usb chargers or player specific cables, as well.
You can install these behind the dash and install a plug in on the dash.
Then you just tune the radio to the "new" channel and you have all the normal radio functions provided by your system, bass, treble, fade, ect....
Outside of changing the Radio to a newer model with bluetooth or an AUX input the modulator would be the simplest unit to use.
And a word about bluetooth, playing audio via bluetooth will drain your phone or player's battery pretty fast, so also get a car charger if you decide on bluetooth