Electrical/ relay problem nearly caused 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.

That wire leading from the battery should be a fuseable link, meaning that that piece of wire will act as the fuse and melt breaking the connection if there is a problem. Can you ohm out the glow plugs, they should have very low resistance but not appear as dead shorts. I had a similar problem years ago replacing glow plugs in a VW, they had a plating on them that caused them all to ohm out as dead shorts, I bought another set, identical except for the plating and I stopped blowing fuses and they worked fine. you can open up the relay and check to see that the contacts are good and operating when the ignition is first turned on....