Um, the unit you linked to here is for when you don't want to buy a tuner - it is used to interface to a Sony stalk remote over the serial port. In the US at least you can buy converters for some cars that make the steering wheel buttons behave like a Sony stalk controller. Then you can plug the converter into your Sony head unit (or empeg tuner) and continue to use the steering wheel buttons.

(list 6284, Mk1 S/N 00299 4GB blue [for sale]. Mk2 S/N 080000094 26GB blue)