Traditional coachworks in California did the conversio
See page 10 'Lost and Found' in the August 2011 Hemmings Classic Car magazine. A Cadillac Mirage was the only one built by a secondary coach builder accepted by Cadillac. You could buy a Mirage from your local dealer and have it serviced and warranted. Other makers did not enjoy the benefit of being a Cadillac accepted builder/modifier. There were several other builders and maybe some locals that we don't know about. This same exact car is pictured in the HCC article.
Cadillac never called these pickup trucks. They simply referred to them as pickups. You may run across one with the quarter windows still in place over the sides of the bed. The true Mirages also had a small door under the quarter window that would open and let you place your golf bag behind the seat without opening the driver or passenger door. Not all Cadillac pickups were created equal. You will see a Mirage in Hemmings from time to time. They are never in my price range.
Mike