Sa accessory wire po found in the ignition switch. Usually color yellow po yun wire na iyan. I suggest letting a knowledgeable car electrician car audio shop do it for you. Pwede din mag lagay nang toggle switch in between the positive wire where the radio gets its power. Problem lang dito medyo hassle kase you now have two switches in order to turn on your radio.
Anyway, i'm sure kayang kayang na nang mga car audio shops gawan nang paraan iyan.