I believe they call it a CAN bus that connects the CD Changer, Ipod Kit and Sat Nav all together the lead I got came from another link on the forum and saved a fortune over the original honda one.
Its to allow an ipod kit to be attached if you have the sat nav installed (a Y splitter)
HON03-F/MM - PIE HON03-F/MM (08A31-0F1-000) 2003-Up Honda Y-Adapter Cable - Accessories - Precision Interface - Discount Car Stereo.com - Detail