[Hot Tabasco]

Who do you think is behind the voice on the car radio?

Well, it didn't sound like Wolfman Jack or Casey Kasem so it's likely someone holed up in an FM radio station(it looked like an FM station signal on the radio) in a nearby city.......and it didn't sound very clear so it might be quite a distance away.

[ConjunctionJunction]

On last night's Talking Dead, producer Gale Anne Hurd gave us more info on the sanctuary and how it relates to the voice on the radio. "Those of you who've read the comic book might have an indication of what it could or could not be, but it offered sanctuary. As we know in this world sanctuary could be a very complicated thing. It could be someplace you want to go and really is what it says it is, or maybe it's not quite what's promised."

Not very helpful. But the producers of TWD have announced the casting of some new characters, so maybe the voice on the radio is related to them?

Highlight if you want to read spoilers: The Walking Dead recently announced that they have cast 3 characters from the graphic novels: Abraham (Michael Cudlitz), Eugene (Josh McDermitt), and Rosita (Christian Serratos).

In the graphic novels, Sanctuary is the home base of a group called The Saviors, and The Saviors are led by a very nasty bad guy by the name of Negan.
/end spoilers.