I am under the understanding that CONVERTIBLES (both 67 or 68) did not use a seatbelt retractor on neither the front or rear seats.
Fastbacks and Coupes used a retractor on the outboard locations, only on the front seats. (deluxe or standard belts)
Many Mustang convertible owners used the "universal retractors" available over the counter at Ford. (made by borg-warner I believe) They slipped over the belt in the middle of the belt and kept them coiled out of the way as not to drag them along the roadways when commonly not used back through the 60's till 80's when seatbelt laws came about.
Well, that has been my understanding, maybe others know differently.
Richard