'74 Duster Radio

i have a 1974 plymouth duster that has factory radio, has 1 speaker in dash and two in back window, its AM , but came from factory like that, did they use different speakers on AM versus AM/FM,, if you want fm added to your factory am, plus mp3, or your FM converted to stereo, and mp3 added ,contact R & B vintage radio in MT. Juliet Tn.,