I think there is a procedure for doing it try this?
Installation Tip - Steering Control Interface Adaptors
This page gives installation advice about how steering interface leads work for installation of head units.
Installation Advice > Steering Control Interface Adaptors PC99-PIO
Important Note:- This lead is only compatible with Pioneer head units up to 2004 including the following, AVIC-XI, DEH-P1500, DEH-P1600, DEH-P2500, DEH-P2600, DEH-P2700, DEH-P3500, DEH-P3600, DEH-P77MP, DEH-P7600, DEH-P8600, DEH-P80P.
For the PC99 range of Steering control leads, we offer the dedicated head unit adaptors such as PC99-PIO. These have to be installed in a set order to ensure the head unit recognises the new leads are attached.
Please follow this routine when installing PC99 leads.
1. Link the Pioneer interface to the PC99-X## series lead.
2. Plug the main loom (not the jack) into both the vehicle and the head unit.
3. Power up the stereo by ignition and seek then store a radio station (this wakes the unit up)
4. Switch the stereo off by ignition.
5. Plug in the jack
6. Switch the stereo on by ignition and wait up to 20 minute for audio to come through before testing the remote controls.
7. If in any doubt that this has not been done in the correct order, the loom must be disconnected from the vehicle, the jack disconnected from the stereo and the process started again.
This system seems to be needed on new (2005) Sony's, JVC's and Alpines. Due to the higher technology employed in these units, they operate more like computers then audio systems, so the addition equipment such as CD Changers, Auxilliary inputs and Steering remote control devices, need to be 'found' by the system after it has been awoken as just a stereo. Future instructions supplied with the leads will cover this.