Engineer Sb. You should be telling us about the procedure then.
Hats off to @zunny bhai who has nothing to do with electronics but did a fabulous DIY!
These head units have got proper dedicated connections for these multimedia buttons. All you have to do is connect the right wire from the clock-spring to the right pin on the head unit. There are people who have connected these buttons to the units that don't have dedicated pins by playing with the resistance every button gives.
Anyways, contact Zammurd @ 0300 5277137 in Blue Area. He will take 1500 - 2000 PKR and will do it for you.
P.S. I am also an Electrical Engineer with Masters in Telecommunication. Therefore, I said it's no rocket science.