If you are having troubles with your old car -like me- you might want to add some features like automatic lights, electric windows or functional steering wheel.
In this project, I will share my experiences about controlling my car stereo over infrared signals.
Infrared signal part is easy. You can just use IrRemote library to learn remote control codes and use it on microcontroller.
Magic trick is using horn relay connection to power our circuit. There is gnd connection from steering axle and positive terminal is coming from relay.
You can not use this controller only while horn is active :)
You must add capacitor and diodes. Otherwise you will fry your board on first horn