I remember Frigiking, based in Dallas, and when a friend took his new '68 GTO to K-Mart to have one installed. He stupidly bought a new car without factory AC and right away had a Frigiking installed. It did cool. It would blow snow too which seemed impressive at the time, but it was probably just low on R-12.
Good article about add-on AC from Popular Science I came across looking for info about Frigiking....
https://books.google.com/books?id=Ey...dallas&f=false