I just picked up the request on uk.rec.caravanning and thought I'd throw my 2p worth in.
I'd be lying if I said I did it myself, but I participated in the fitting of the towbar to my current car, a 2002 Laguna 2.0
First thing is first, you need to count the pins in the existing socket. If there are only 7 then you have only the basic towing electrics. If there are 13 then you may be in luck and only need an adapter cable. Most caravan dealers stock these as a standard part these days, but you may still encounter problems with caravan battery charging and fridge operation because of differences between the UK and European wiring standards.
If you only have 7 pins in the current socket, then the bad news is that the supplementary socket wiring is the hardest part of the job. I couldn't find a route through the inside of the car, and ended up running conduit the full length of the underneath. It's tricky avoiding the back box on the exhaust when it takes up the full width of the car, but it's 'doable' and this problem might be specific to the 2.0
The easiest way to sort out the switching is to use a 'voltage sensing split charge relay' (www.towsure.co.uk
if you can't find one locally.) Alternatively, you could use a conventional relay, switched from the ignition or alternator. The latter is much more hassle, and on some cars (particularly certain French ones) it is possible to damage the alternator.
Don't worry about multiplex wiring: The 2002 Laguna doesn't use it. As far as I can work out, you don't even have bulb failure monitoring to worry about (a step backwards from my old 1998 Laguna 2.0 RXE, but there you go.) None of this is relevant for supplementary electrics anyway: You just need a 12v feed straight from the positive clamp of the battery via a suitable fuse, and very thick wire to avoid voltage drop. Somehow you need to get this wire into the boot (as I mentioned before, under the car inside conduit seemed to be the only practical solution to me.) The rest is just a case of following the instructions that come with the split charge relay.
If the previous owner has only used the towbar for small trailers, they may not have bothered wiring up the rear fog light connection. You're lucky in having the sport tourer, as the reversing and rear fogs should be easily accessible in the main lighting clusters - if you have the hatch, you need to remove the left hand boot interior panel and go digging for a multi-pole connector. If you need to sort out the rear fogs, then you might as well do the reversing lights at the same time (even though they're not legally required.) This should be wired to pin 1 of the supplementary socket.