You've got a MK3 so it mightn't be the same as for me... I just put a cigarette socket in the boot, and i used the power from the boot light feed, and from the tailgate (to light it) Pretty easy for a MK2. I used thinish single core wire, and it wa fine. I now have three! I fitted one in the front using the radio supply, with household plug wiring, then fitted a centre console that had one in.
Just have to say watch what dissipation of power is though as you could drain your battery, although if you were worried about that, just put an op amp with potential divider on the non-inverting input, and it'll only work when the votage is above a certain amount (eg 12.2) thus giving you a cut off