there Isnt one AFAIK, what do you need it for?
If its really needed you need a NC (normally closed) relay, it passes power with no power to the coil, activate the coil, it opens the switch.
I suspect what OP actually wants is a permanent live for the memory backup on the radio - which is not what he asked for. That being the case, the nearest brown wire should do with an inline fuse assuming that the power required is negligible.