Steering Wheel control Primary harness adapters convert the commands from your car's steering wheel controls to a code that an aftermarket radio can recognize. A patch lead is also requires for SWC type systems based on the brand of head unit you have. for more details check out our you tube video. http://aerpro.com/video/control_harness_installation CHAD3C Quadlock Half Amplified Bose AUDI A3 2003 UP (CONCERT II / SYMPHONY II) AUDI A4 2005 UP (CHORUS II / BLAUPUNKT) AUDI TT 2006 UP (MATSUSHITA / GRUNDIG) MODELS WITH CAN-BUS + QUADLOCK