I don't recall ever seeing any in-service Mustangs with tinted rear windows but you never know. There's a good chance yours served the in a desert area and sometimes this was done to units in those areas to help keep the interiors cool. It was also a popular "mod" in the day with civilians though so the odds are it was done by a subsequent owner. Ditto with removing the rear seat brace...probably done to stuff more "beats" back there (sub-woofers and all that crap). The CHP did not remove them as part of their upfitting so I'd find a replacement and put it back in.
There is another tantalizing possibility here too. Over the years, the CHP did convert a number of Mustangs that were fully or almost fully runout to undercover investigation units. This typically involved repainting them in a popular civilian color and outfitting them with nice wheels, sound systems and other goodies to help make them blend in the mainstream of the day. But to prove your car was one of these, you'd need some good documentation like the unit's Assignment Card.
Keep posting progress pics!
