Lionel battery harness for 6-11428 railsounds

[gwalk2010]

I have a Lionel switcher 6-11428 with railsounds. When I turn off the track power the sound stops instantly. I saw on a Lionel instructional video that says you can buy a wire harness so you can put a battery in to prolong the railsounds and have a slow shut down of sounds. I contacted Lionel they said it is available but they do not have instructions on how to install. Can anyone help.