In-Car Charging of iphone via ipod touch connector 3G

I have a VW Golf which was fitted with an ipod touch connection as an after-sales accessory. The installation consists of a single lead coming out of the dashboard, terminating in a ipod connector and then linking through the in-car radio and speakers for ipod output. I have now purchased an iphone and hoped to use it in the Golf using the existing ipod installation. However, when I connect the iphone, a message is displayed saying charging is not supported with this accessory followed by a new message this accessory is not made to work with iphone. Does anybody have a fix so that I can use my new iphone on this ipod installation+
Macbook Mac OS X (10.5.6) Nil