Urgent help needed

[Terrano03]

Hi, I have only had my terrano 3lt diesel for a few weeks, love it as family had one for years. Any way onto my problem, I tow my caravan so used the terrano this week we are in Derbyshire, however I had only the normal black plug for electrics, and my caravan has to have the gray 12s also. I have gone out today and got the cable, bracket, plug and a 12s smart dual relay. But I can not find on the site or net where to wire into the back of the terrano. Please if you can help, as the caravan is my life line as I take my 5 year old away as I gave him my kidney 3 years ago, and it is our only safe way of being out and about, the terrano allows us 12 month towing and all we need. But I need to attempt this tomorrow as need to drive again on Saturday morning. Please can you assist. Kind regards