Every Bronco I ever saw had it on the driver's side. Until I bought my '71 which had it on the passenger side.
Since then I've seen many on the passenger side as well. Not sure what's up with that, but it's a good question. And a great suggestion for a thread similar to the "P's and Q's" mentioned.
My first assumption was that it was added. But the truck did come with a radio from the factory and the hole sure looked like a factory made hole. I was always impressed with the hole because I figured it must have been done by someone after the truck was built.
Now I'm not so sure.
But I will say it did not have the factory type of antenna. Just a standard single mast flat base that never fit perfectly. I've had several antennas there since, but eventually figured to put it back to the driver's side when I found a stock one.
Now that I've waited 40 years, they make them new so I don't have to scrounge the yards any more!
Paul