As per title, Disco 1 300tdi. Where would be the easiest place to tap off a 12v feed which becomes live when the ignition key is switched to the 2nd position?
There is No 1, its a spare terminal on the fuse box depending on your vehicles spec.
So no need to go cutting into other "unknown" circuit wiring.
I use the supply for my Bluetooth car kit, the blue wire shows the terminal.
The lower terminal No.2 of the two is an earth, but that is removed on ign. position 2.
Or the feed to the cigar lighter....but as mentioned above, take a (Fused) feed from the battery to a suitable relay, & switch the relay from the "ignition-fed" source.