here is my answer to another question about the same problem:
it's just because of the cable you use. You should either use a "patched" scart connector, or use composite output.
Check this out :
this nice tutorial explains how to do such a "patched" scart cable by yourself.
BTW, you can but such a scart connector but it's a try and miss, usual SCART connectors do not have this "patch" since it's out of the standards.
but never mind, just grab a soldering iron. I did it, it took only a few minutes, and still use it Smile