Great research Mark. One small point of information. I just donated several photos of the JFK Ghia that I found quite accidentally through the John F. Kennedy Library in Boston. At least a couple of the photos (and several more that I didn't purchase because it showed so little of the Imperial) clearly show Jackie Kennedy and her family entering and exiting the Ghia at various points in the Funeral. One photo taken on Constitution Avenue (which I didn't purchase), clearly shows the entire family in the Ghia. One story I've read, which would seem to answer the question, is that Mrs. Kennedy did get out of the Ghia, and walk some distance (many blocks..., half the route), but the Ghia was indeed used as the lead car in the JFK funera, holding Mrs. Kennedy and family for some time...l.
Back to the Home of the Forward Look Network