I nominate the "NO RADIO" plaque that adorned the rear-side window of many a car during the '70s and '80s.
In the 1970s and 1980s New York City appeared to be in a downward spiral - the narrative was dominated by crime, filth, blight, drugs, bankruptcy, Son-of-Sam, squeegee men, graffiti . . .
Any car parked with a visible tape deck was almost assured of a break-in. Motorists took to placing "NO RADIO" signs on their cars to forestall this.
This would be, as a single object, a powerful iconic representation of the mood of those dark times.