If you've correctly replaced the cable, then either the antenna or the receiver is faulty.
I'm by no means an expert, so the following advice may well be utterly ludicrous, but here's what I'd try in the same situation.
The first thing I'd do is re-check all of the connections. If you're lucky enough to be able to find a replacement GPS antenna at the scrappy's, then try that too. Though this is a real long shot, and if you do find a car with one fitted, you'll probably be grabbing the rest of the sat-nav system too unless someone's beaten you to it. I'm not sure about the Laguna I, but a lot of the Laguna II models have a mobile phone antenna that looks identical to the GPS antenna except that it doesn't have the letters 'GPS' printed on it. Make sure you don't end up with one of those, as it's unlikely to pick up the correct frequencies.
The second thing I'd try is to beg, borrow or steal a re-radiating antenna. As a last resort, you could risk buying one (they're about 45 quid) but there's a good chance it won't solve your problem. If you can find somewhere that'll give you a refund if it doesn't sort the problem, then that'd be perfect. Alternatively, you could
it as 'used only once' and hope you get most of your money back.
Go for one with a short range (i.e. 50cm or less) as the longer range ones can apparently give feedback problems. Something like this should be perfect http://www.gpsw.co.uk/details/prod444.html
Put the antenna on the outside of the roof for now (you can find somewhere better to put it later) and then pull out the sat-nav control panel and stick the little relay transmitter as close as you can to the GPS aerial socket. If it works, then re-fit the system with the re-radiating antenna in place and just use it like that.
And if that doesn't work, my best advice would be to leave it and (if you can't live without sat-nav) buy a stand-alone system. Even if you get Carminat working, it'll cost you over 100 notes to get the up to date map disc, and Carminat really isn't a very good navigation system (I know, I know, I'm starting to sound like a stuck record.) You'd be far better off putting the money towards something like TomTom.