What does "[got] a bird-dog on" mean?

I have recently discovered the, in my view, rather splendid song Guitar Town by Steve Earle (released 1986). It contains the following lyric:

Hey pretty baby are you ready for me,
It's your good rockin' daddy down from Tennessee.
I'm just out of Austin bound for San Antone
With the radio blastin' and a bird-dog on

Wise words, for sure, but this small town middle-class boy from the UK does not understand what it means to "have your bird-dog on"!

I have searched Urban Dictionary and other sources located through Google, and they all tell me a "bird-dog" is a man who chases other men's women, or sometimes just a man who chases any women. None of those explanations seem to fit with Steve Earle's meaning in Guitar Town. Unless he means that he is in the mood for chasing women, but the rest of the song is about driving from town to town putting on shows...

See also http://youtu.be/otTW0JczoGQ at around 0:19

According to Wikipedia, bird-dog:

• A bird dog can be noun which means a radar detector.(Taken from Urban dictionary) Example from Steve Earle's song "Guitar Town": "I'm just out of Austin bound for San Antone With the radio blasting and the bird dog on"

Bird dog: radar detector

( from www.radardetector.net)

The Bird Dog was a local radio DJ in Austin area of Texas. I asked Steve Earle this very same question a few years back in Perth Scotland.