Great info and detail. I have been involved in Radio campaigns and find that they can work for small business... as long as you approach it the right way. When you need to get that 'bang' for your buck, your offer has to be really good. Let me explain.
If I were in the automotive business, I am not sure if I would waste my time on the radio advertising my services... because your market is narrow. People are not going to drive all over for general service. They typically go where it's a little more convenience.
On the other hand, tell people you're giving away $100 bills... the line up will start almost immediately (so people do listen
The point is, you can use radio to generate new leads. Make a great offer... tie it to a free give-away and collect names and numbers. That will work for a lot of different sectors.
Hope this helps!