An armoured convertible? Seems like a waste of time

Reinhard Heydrich, Himmler's right hand man and the only major Third Reich figure to be assassinated during the war was killed by commandos while in a convertible. Thing was, had he been in an armored convertible he likely would have survived because the bomb that was thrown landed at either the back of the car or the back seat area (I can't remember which). From what I recall reading about the attack was that an armored version would have had plating in or on the back of the front seat. In the the attack, when the bomb detonated it blew pieces of the seat into his body leading to his death several days later.