Boot ignition live wire for lighter socket

[C5Alan]

I want to fit a lighter socket in the boot but only want it to be live when the ignition is on. Is there anything in the boot aera I can splice into easily that won't cause the computer to freak out any time I think about doing anything to this car it seem you need a lexia to do it.

If all else fails I might just splice into the towbar electrics to use the split charge relay for the caravan plug and fit a second battery to run extras off but dono where to fit it out of the way. It was a great way to run a fridge in the boot and not worry about a flat battery but what's the point in this weather