1985 300x n/a seat belt retractor

hi, i just bough a 1985 z31 and the right hand seat belt does not retract all the way. sometimes it goes up half way and then sometimes it doesn't go back up at all. I'm guessing this has something to do with the seat belt retractor. I am having trouble knowing what parts to look at. Not sure where to look. and not sure if i should replace the whole seat belt assemble or just the retractor. if you thing another part of the seatbelt assembly is broken and not the retractor please let me know.
thanks.