There is another way without disconnecting it....if you happen to be using an iphone or pod touch, you can jailbreak it an there is an app that removes the restriction on using the iphone music interface while plugged into an accessory.

Can't remember it right now, but if you are interested I will look it up for you....

I'd be interested. I normally use my iPod classic but I have an old jailbroken iPhone I could use

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OK, so if you have the phone jailbroken already - I assume that you will have Cydia installed already - open Cydia and search for an application "NoAccessorySplash"

This will allow you to use the iPod interface on the phone while it is plugged in. It will also get rid of the annoying message when you plug it into the car that the accessory is not supported.

Let me know if you have any issues!

Have fun.

Cool, will have to have a go at that once the GF upgrades her 3GS leaving it to me