This might be useful to someone, or may be common knowledge so here goes,
I've spent a fair bit of time looking for a head unit that sits nicely in the C's dash (many stick out too far),has reasonable sound quality and matches the colour and brightness of the dash lighting. Found the VW gamma iv does the job pretty well - see pic.
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These are available in 2 versions (green illumination and blue screen, red buttons for that updated look) and cost pennies as it appears VW are getting rid of their refurbed stock. There's currently one on ebay (blue/red) factory refurbed with code for £7.00 buy it now.
They're not the last word in sound quality but are not bad and have an old style 10 pin iso plug for CD changer/Line out - you can still get Connects2 CD changer or Aux/iPod leads. They plug straight in the the C's loom (no need to reverse Perm and Ign) and slot straight into the dash with no cage as VW intended. Just make sure you get removal keys as they won't come out without them.