How to parse through FCK iFrame ID button was pressed from

I'm trying to write some code for my module see #603404: New Interface - Testing Needed if you want background info.

I'm trying to get back to the FCK object. In the past when working from a pop-up window I would use the code:

var FCK = top.window.opener.FCK;

However instead of coming from a pop-up window I'm now making a request from an iFrame. So I need to go back to parent window and into the FCK iFrame to fetch the FCK object. This code works:

var FCK = parent.oFCK_1___Frame.FCK;

But i'm assuming the code is coming the first instance of an FCK iFrame, which won't always be the case. How can I know which one I'm coming from to parse information back?

Cheers in advance