Since buying the car I've never plugged anything into the 12v socket in the boot section. Anyway, I havr a few devices like chargers and coolers that would plug into it, so I decided to give them a go. Turns out that nothing works.
I checked the 30amp fuse and it was blown.

I replace it with a spare, and CRACK , blue spark and that one is fused immediately. Tried one more and got the same. Short is somewhere but where do I start looking?

Nothing was plugged into the socket by the way.