91LegacyL

I just bought a 91 Legacy L wagon a couple of days ago. It has rear shoulder harnesses but no headrests. I'm wondering if I have to get a new seat back with headrests or can I pull the plastic tie down clips on the top of the seatback and mount headrests there, if they have the single flat bar post unlike the double posts of the front seat rests. I don't think I can. Anyone know when headrests were an option or standard on these cars? I'm thinking 1996. Anyone know how new of a seatback with headrests will fit the 91 or the year range that the rear seats or seatbacks are interchangeable? Any good sources for these items? Thanks, Dave near Seattle, WA