I found this via craigslist from a previous 99 owner who bought it from the dealer around 1978 but never installed it. There are a few variations, some including a speaker mount, some including the ash tray. Most folks want the Turbo center console which made its way into the model 900.
In the mid 70s and earlier 99s the radio was mounted in the center of the dash where the REAR DEMIST/Fasten Seat Belts panel is and the speaker was in the dash pad. In fact, you can see after I removed the dash paid there is still a perforation in the sheet metal for adding the dash speaker.
Here is what I had before:
Here is the new setup:
