Just connected a rear facing dash cam and found a fuse in the rear fuse box next to the spare wheel labelled "FB1". This is only active when the ignition is on and normally feeds the reversing alert system. Used a piggy-back connector so that each circuit is protected seperately. I used one of the nuts securing another electric box as an earth. Hope this is useful as I've seen a few posts asking about ignition switched actives for this purpose.