OK, I'm going to have a go at this, but I may well be going down completely the wrong track.
OK, the Scart on the left is the one we got with the GC. That works just dandy on both the GC and the N64.
The Scart on the right was bought off the internet. Advertised as being able to cope with GC, N64 and I also believe PS, this is the one I had to mod.
I found the same problem as Bluelagoo when I plugged the Scart in, sound, but no picture.
Don't ask me how it works, but the simple removal of a resistor overcame the problem. I have two N64 systems set up currently, both with these Scart leads, and both had to be modded to get a picture.
Hope this helps. Failing that, we can look at other options later
Squigs.
Edit: Just so people don't think I'm referring to modding the N64 unit itself, the resistor is in the Scart lead. I'm adding this as I know people on this site don't like the idea of modding systems etc.