[Aly]

It's kinda easy.
Well you can get the player's car ID and then make a loop to check if there are another players in the same car and to be a little bit better make a variable "persinthecar"(example) and everytime he found a person in the same car persinthecar++; when persinthecar reaches 3 use the function break;
Be careful to avoid in the loop the player who changed the radio: "if(playerid != i)"

It will look like:

pawn Код:

new persinthecar,newcar;
newcar = GetPlayerVehicleID(playerid);
for(new i=0;i<MAX_PLAYERS;i++)
{
 if(playerid != i)
 {
  if(newcar == GetPlayerVehicleID(i))
  {
    //Change the stream....
  }
 }
}