Bumping an old thread, may be useful for future references.
Often times installing steering wheel audio controls involves going two routes:
1. The OEM way. This would involve purchasing the steering wheel with audio controls or just the audio control module itself from higher end models of the car or from a different regional model of the car in question. It would also require a new clock spring to facilitate the extra wiring. This can then be mated to one of the many available aftermarket solutions for adapting steering wheel controls to an aftermarket stereo such as the Metra ASWC-1 or the PAC SWI or sometimes it may be a direct plug and play with the factory stereo.
2. The aftermarket method. If you already have an aftermarket head unit such as a Pioneer, it might be possible to buy one of those infrared steering wheel remotes. In my case, I have a Pioneer P4000DVD stereo and as an easy addon, I was able to purchase the Pioneer CD-SR100 steering wheel remote control which attaches to the steering wheel by using an included flexible plastic strap. However, this means you may have to use the remote upside down as they are designed for the American market generally where cars are left hand drive. I had to open the remote and enlarge the openings and then install the remote upside down on my steering wheel. Other than the fact that its upside down, it works great and I never have to take my eyes off the road to adjust my stereo.