steering wheel buttons

[MPS]

I have fitted a set of early [2005.5] steering wheel buttons, the ones with the track change and volume buttons on the right side, all the buttons work apart from the source select button on the left. I have it fitted with an RCD510. The mute on the left works. Is there any measuring blocks I could check it with? Could it be a broken wire maybe?

Any ideas guys/gals?

Thanks