Electrical problem that nearly caused a fire!!!

Hi folks, my '97 300tdi Disco wouldn't start (long story) so I've been checking different things out on it trying to get it going and I discovered 1 of the glow plugs was faulty and didn't heat. I bought and fitted 4new one's (definitely the correct one's) and when I turned the ignition on the lead linking the glow plugs started to melt and had smoke coming from it! Obviously I switched the ignition off and when I looked under the bonnet the brown and orange wire coming from the positive battery terminal had melted and had smoke coming off it too.
I replaced the section of brown and orange wire and checked it with a multimeter and it's fine now. lt runs up into a relay mounted behind the expansion tank and from what I can make out the wire that then goes over towards the glow plugs is black and yellow but testing this using a multimeter shows that there's no voltage in it whether the ignition is on or off.
Is this the correct wire and should it give a reading when the ignition is turned, is the relay for the glow plugs?
The glow plug fuse in the fusebox is fine but the glow plug light on the instrument panel no longer comes on when the key is turned so I'm guessing it's the relay that's bust. Can anyone shed any light on this?
Cheers, Tam.

well, thats a dead short somewhere, i dont know about glowplug leads, abut is it just a (+ive) lead then earthed back via the block, or does it return on the same lead?

Sorry for not getting back sooner, been working away from home.
There's just the one lead so it must be earthed through the block I guess.
Cheers, Tam