The suspicious broadcast locations all come from the Ghost town map layout (though it does not say this in the quest text). They're all in buildings around the map.
For people having issues with the save the world mission where you're looking for bunkers.
(This is a repost of the one from a little while ago because I had to run it back and get a picture of the third type of bunker)
Generally they're found on the outskirts of the maps near the cliffs. There are three different varieties that I've found and from what I can tell, one spawns on every Thunder Route 99 Map.
For people having issues with the save the world mission where you're looking for bunkers.
There's also another one with a round roof over the top of it that looks kind of like an airport building.