muhibmc, who described correctly the whole process for upgrading to backlit, wrote at a certain point:
"**there will also be a clear film right under the black one, you do not have to remove that one."
well, after performing the same operation with success, I can tall you DO have to replace the original clear sheet instead, wich you find behind the keyboard, with the new "white-dotted" one coming with the backlit spare part.
I think you *may* even leave it and install the backlit components behind it as well, but I don't think the original backlit Macbooks feature two clear plastic films one behind the other. I guess they have the backlit only.