There can be multiple type of issues when your car stereo system turns on without using key.
First problem which most of the time happens is the yellow constant wire and the red ignition wire internally touch themselves and that causes the car stereo to turn on without key. The constant wire carries a direct live current from the battery of the car which helps it to remember the radio internal settings and the red wire is the ignition wire which sends the signal to the radio to turn on when the key is inserted, when these both wires are attached together then the key feature is disabled and the radio turns on. If the car manufacturer has designed its stereo to turn on when you unlock the car, then also this problem might occur. Modern cars uses interface to turn on the radio, which does not require any key to start up the radio. Sometimes the interface crashes and this type of problem starts to happen. The interface does not have any sort of wires which controls the car stereo, it sends a bunch of signals to start up the electrical equipments when you start up the engine of the car. So when any problem happens to the interface, then the radio starts to behave abnormal like automatic starting up or high volume when you start up the radio.
Incorrect installation of the radio wires is the main reason for the car stereo turning on without key. Wrong wires connected to wrong path will result such issues. Aftermarket car radio are very easy to troubleshoot if such kind of issues arises. Sometimes the radio by itself has some internal problems also which causes it to start up without any control over it.
Interface controlled cars are complicated to check the actual root of the problem and the cost of repairing them will also be really expensive as the interface itself is very expensive stuff. If you have a wiring harness in the radio of your car, then you easily buy another wiring harness of same type and replace it which will resolve the issue.
How the wiring mechanism works?
The wiring harness of the aftermarket head unit has several wires of different colours. These colours represent the functionality of the wire which will be done by the head unit. Incorrect wire installation will lead to improper working of the head unit as well as damage the equipment as well. If you are installing a aftermarket head unit in your car you will need a proper dash kit which will work as the frame for the head unit and the wiring harness which will be used to connect wire to the stereo.
First step to wire any radio is to figure out what car you have and determine what you need, a wiring harness or an interface.
There is a big difference between both of them, interface is used in modern cars and mostly installed in cars which have factory amplifier system in the car. Interface is used in modern cars because they have data bus systems which sends series of signals and the data bus decodes those signals and then tells the radio to turn on by providing the power. The wiring harness requires wires to be connected to the wires of the car and the functions will be done. The modern cars do not have a wire which turn on with the key that is why you need an interface to be in installed in the modern cars.
Wire Functions in wiring harness of car stereo
Red: It is the accessory wire which tells the stereo to turn on when the car is put to the ignition mode. This is one of the main wire which start up the head unit.
Yellow: This is the Constant wire which is always on. It is used for the stereo to remember the settings like time, sound settings etc. This is one of the main wire which start up the head unit.
Black: This is the ground wire of the head unit. If you are installing after market head unit in your car, then do not use the stock factory ground and make a fresh ground for it. Installing aftermarket head unit to the stock ground wire can introduce some issues like noise in speakers.
Besides these 3 wires, below listed wires are not used for working the head unit. If your head unit do not start then check these 3 wires only.
Orange: This is the illumination wire.
Orange with black strip: This is the Dimmer wire which sends signal when to dim the lights of the head unit.
Blue: This is a Remote Turn On wire for the aftermarket amplifier which sends signal to turn on. This is also Power antenna wire which is used for connecting the external antenna in the car.
Reverse wire: This is the wire connected to the back camera of the car to activate it when the reverse gear is used.
Just match the wires with the colour of the wiring harness of the radio and you are done. If you face issues like not starting up of head unit, then check properly that wire is connected and to the right wire as well.
Purple: This is the right rear (+) speaker wire.
Purple with black strip: This is the right rear (-) speaker wire.
Green: This is the Left Rear (+) speaker wire.
Green with black strip: This is the Left Rear (-) speaker wire.
White: This is the Left Front (+) speaker wire.
White with black strip: This is the Left Front (-) speaker wire.
Grey: This is the Right Front (+) speaker wire.
Grey with black strip: This is the Right Front (-) speaker wire.