I take it the original AM radio is missing and you have an aftermarket unit in its place?
Are original speakers in the doors, and has the dash speaker been left in place unplugged?
Your door speaker setup is all original parts because none of it was available repro in '79 of course.
It's possibly a dealer install from new, but could also be an owner install using original salvaged parts.
You could always add in an original AM/FM unit.
Nothing wrong with accessorizing a car in my opinion, but if you want it all assembly line that can't be faulted.
I added all that same stuff using original parts and have an original AM/FM radio installed and an original AM 8 track on the shelf since it's less appealing to me.
Car makers published big thick books of dealer accessory installation instructions for the shop to install items sales staff pushed customers to buy to improve the plain Jane lot car they were buying, or to equip cars the way a customer wanted since they missed out on, or didn't want to wait for a factory order.