I am not going to answer your question because I don't know. But I suggest
you advise your grand son to insulation behind the rear license plate,
between the deck lid and its reinforcement bars, under the package shelf,
between any metal surfaces that are within 1/4" of each other, and attached
insulation to any panel that can be flexed by 100 pounds of force.

Normally a car with these type speakers can be heard half a block away
because of all the ratteling sheet metal.

Well maybe my suggestions would take all the fun out of having a car that
shakes, rattles, and rolls when the radio is on. :~) Savor the time you
spend with your grandson. I hope to have one in about 10 to 15 years.