By default the player is controlled by the following keys:

[keys]
key_on=123 //F12 - toggle player
key_prev=120 //F9 - previous track
key_next=121 //F10 - next track
key_pause=119 //F8 - pause
key_shuffle=122 //F11 - shuffle mode (the track will be played randomly)
key_end=35 //END - quick exit from game

[playlist]
C:\Users\13\Music\OST Midnight Club L.A\Electronic\Midnight Madness.mp3
C:\Users\13\Music\GTA4 TBoGT OST\Vice City FM



Install:
Copy the file radio.asi to the game root directory and radio.ini to CLEO folder.
Open the file radio.ini, find the section [playlist] and write the full paths to the MP3 files you want to listen to. Each line should have only one path.

Thanks Seemann for San Andreas version of this plugin.