When I had my Laguna, one of the parts I wanted to integrate with the CarPC was the manufacturer’s steering wheel controls. It turned out to be very simple using a £10 USB Numpad from Tesco.
Here’s a picture of the circuit inside the control arm
You’ll notice not many wires and a rotary position switch. I then traced the cable out to the stalk back to a connector to place a multimeter on the output to see what was what. It transpires that the circuit tells the stock Renault head unit which button has been pressed via a key matrix circuit. This image helped me understand how it worked and this Wiki page helps explain it more.
With the knowledge that it was just like a keyboard I bought a USB Keypad like this one (mine was silver if you needed to know) and pulled it apart.
It was then a simple case of soldering the wires of a floppy disk cable to the input of the keypad’s circuit like so:
The wires were very fine, so were soldered and then hot-glued and wrapped in cardboard for protection (I was in a camp site when I did it and had to McGuyver some parts). Then the floppy cable connector, which had the same pin pitch as the Laguna’s stalk connector simply plugged in like so: