Over 30 years ago, when I had decided that I was either going to add an AM - 8 Track or a Hang-On 8 - Track to my '67 (and before I had a copy of the Ford Instruction Sheet), I too wondered how the Door Speakers were to be installed. No speaker grills for '67 were shown in the Ford Parts books.
I contacted Bob Perkins, who, even back then, had the reputation of an "authority" on Mustangs. Bob told me that cars that were ordered with the AM - 8 Track Option and Standard Interior, came with the Deluxe lower door grills from the Factory, and without the door courtesy lights. He also noted that the majority of cars he had seen with either Dealer Installed AM - 8 Tracks or Hang-On 8 Tracks, also had the Deluxe lower door grills, some even with the door courtesy lights. This does not seem unusual since, as noted on the Instruction Sheet, if the car didn't have the lower moldings (ie was a Standard Interior), the Dealer was to get them from its "stock". Of course, if the Dealer didn't have them in stock, or if for some reason the Owner didn't want (or couldn't afford) the Deluxe lower moldings, it would be easy to substitute 1966 Door Speaker Grills. Likewise, it the Dealer were installing the AM - 8 Track or Hang-On 8 Track, it would be easy to also add, at the Owner's request or at the Dealer's suggestion, the Door Courtesy Lights.
Just for info, I opted to go with an original AM-FM and the Ford Hang-On 8 Track.
Hope this helps.
Bob