Baffled 12v

My best guess is that the missing wires terminate at the trailer plug on the trailer.
You probably will need to run a fused line from the 12V power supply (battery) in the vehicle to the No 2 terminal on the trailer plug on your vehicle. I fitted an illuminated switch on this supply wire and fitted in the cab so that I could switch the power off when parked.
Be very aware that if you don't switch off the power to the van when parked you will finish up with a flat battery, 3way fridges are notoriously big power users.

You will have to dis assemble the plugs to verify if this wiring exists, but it was the most usual way of wiring up car/caravan/campertrailer back when they were new.

It probably would be a good idea to replace both plugs to ensure that everything is as good as possible, and an even better one to take the power for fridge and internal lighting through a 50amp Anderson Plug rather than the trailer plugs.

If you are still confused I would suggest that you take both the camper and the towing vehicle to an auto sparky and get it all checked out.

Disco.

Paul,

Sounds a bit dodgy - "at the back of the fridge is a 12v 3pin socket". I'm not aware of any legitimate 12V 3pin sockets, and can't see any reason why one should exist.

If that's a 3pin socket and looks like a 240V power point outlet, I wouldn't trust it to be 12V. Suggest take it to someone who is competent to check it out for you and tell you just what is going where. It's very bad practice to use 240V fittings for low voltage because of the possible confusion and assumptions that "its only 12V".

Cheers

John