Short of buying an MP3 compatible receiver, what is the best and most economical way to adapt an MP3 player (which I have not bought) to a CD player for the purpose of playing voice audio.
You can buy an RF modulator that goes in line between your antenna and your head unit. The modulator accepts RCA inputs and converts it to FM radio signal which is played through your head unit, much the same way adding an after market CD changer to a factory head unit works.