New Ignition cable run to radio

i have a 2002 X5 E53 and i recently put in a new radio.satnav, when i bought the car the radio had to be turned on and off with the button as the ignition link did not work. Now with the new radio/satnav there is a small light that stays on even when the radio and car is locked and off. this is because the new radio is connected to the +12v but it is draining the new battery when the car is not in use for 2-3 days. I checked the current purple/white ignition cable in the loom and there is no power in it at all. I need to run a new ignition detection cable into the radio but where is the best place to take this from?