Challenger electrical question

To all the electrically minded kinfolk...

I have a MY01 Mitsu Challenger (which I am very happy with, please no comments on my state of mind!) that has a problem with the 12V outlets. For those unfamiliar, these come standard with 3 12V sockets, 2 in the cabin (one is the obligatory cigarette lighter) and one in the back. Well, sometime ago something went awry and only the ciggie lighter worked. Since I was headed up to Fraser, I thought I would get around to getting in fixed (under warranty). They (the auto sparkies) found a broken pin in the wiring harness or something like that. Anyway, everything worked. Loaded everything up and plugged the 3-way fridge into the rear point (yes I know, won’t work very well without a line direct from the battery, but it has worked satisfactorily in the past). Low and behold, arrive up the there with a warmish fridge, and not power to the 2 points again!!! The point is rated to 120W, the same as the fridge. So my question is….what is going on?? Am I simply melting a pin somewhere (aka the “broken pin”) that was found, and therefore should I give up and replace the wiring? Or did they not fix it properly? Any idea what putting in a heavy duty line to the battery will cost?

Finally, anyone have any idea what a second battery will cost to install? I see you can do it, but it will be a tight fit. At the moment I have an overlander 700 in the cradle, any thoughts about replacing this with 2 smaller batteries side by side?

Thanks all