Peugeot 406 Hdi 2002 12v supply to towbar?

I have a 2002 peugeot 406 hdi estate.
There is a 12v supply wire at the rear for the towbar auxilliary electrics. Unfortunately this has shorted to earth presumably blowing a fuse somewhere.
I have checked the fuses in both fuseboxes but cannot find a blown one. The handbook layout does not correspond with the actual fuse box.
Anyone able to tell me if there is a seperate fuse somewhere?