The pictured Top being placed on the JFK Limo is NOT the top that was installed on the Limo at Parkland Hospital. That Top was stored in pieces inside the trunk of the Limo. Said Top was extracted from the Limo trunk and assembled/installed as the car sat in that Parkland Emergency parking stall.
Personally, based on All of the 11/22/63 images of the JFK Limo I have seen, I believe that the window that runs entirely across the JFK Limo, (from door-to-door), had been removed. It could be that anytime they took the top off the the Limo they also removed the window. It could be a vibration issue without the support of the top? Also, if the top is on the Limo and that window is installed & closed, if shot(s) were fired how would a SS Agent such as Kellerman move from the front seat to the rear section of the Limo in order to protect/physically cover the POTUS? SA Youngblood physically did this with regard to LBJ. SA Hill also did this with Jackie who he was specifically assigned to protect. This makes it obvious as to what is SS Protocol regarding shots fired and the duty of a SS Agent with regard to who they are assigned to protect.