I haven't seen the ad, but if the lyrics are as follows:
When the road gets dark And you can no longer see
Let my love throw a spark Have a little faith in me.
And when the tears you cry Are all you can believe
Give these loving arms a try baby Have a little faith in me.
Have a little faith in me Have a little faith in me - etc
It was written by John Hiatt but has been recorded by loads of people as you can see on
http://www.audiogalaxy.com the free (and thus probably hugely illegal) music download site.