Edit data field of an application/shockwave-flash video in javascript

I'm searching for a javascript code that allow me to add this each time an application/shockwave-flash video (youtube embedded) is present on the page. Just take a look at the screen below :

http://goo.gl/HmaHGg (sorry for goo.gl link but I can't upload images)

So I want to add "?rel=0&autoplay=1" to the data field of the video so it can be played automaticly when the page is started.

I've tried to do "document.getElementbyId('myytplayer')" and "document.getElementById('ytplayer')" but the answer is null.

I hope that you know what I mean, thank you for trying to help me,

Have a good day ;)