As I recall, the Dept. of Transportation required 3 point belts for all cars which were not convertibles to have them in the front seats. Precursor to this was the "bulbous" center hub of the steering wheel in '67.
I do not know about 69 Mustangs, but the 69 Cougar still had the "headliner" third belt, so I would believe that the Mustangs did as well. I would take a pic, but it is raining cats and dogs here in San Jose, and Baby doesn't do rain. I will try to post a pic when the sun shines, if someone doesn't beat me to it.
Robert