odd head unit wiring

hi im installing a new head deck for the first time but im stuck, ive determined which cables are for speakers which leaves 4 1 is deffinatly ground and one is the ignition 12v which leaves two one is either illumination or dimmer, with park lights on 11.54V and dimmed beam 11.18, what exactly is illumination the finnal cable by my logic should be constant 12V but it shows 0V-0.2V so im a bit stumped

When you checked the last wire did you do two seperate tests, one with the car off and another with the car turned to accesory?  If it's reading 0V with the car off and between 11-13 roughly with the car on accessory then that's your switched 12V wire. Usually goes to the red on aftermarket head units.