The CD changer in my Premium 7 head unit broke and gave the error CDC Hardware Error. CDC stands for Compact Disc Changer. Every time the radio turned on and I pressed the CD button or was already on CD it showed that error. I did a little research and this happens sometimes with these units.
I ended up purchasing a used Premium 7 unit off of Ebay from a wreak to replace it but it wasn’t exactly the same. The broken unit was a 1K0 035 180C and the unit I got was a 1K0 035 180A. The only visual differences you can see are the VOL and Tune nobs don’t have the chrome and the SAT is instead a MIX button. The A model is not compatible with Satellite radio but other functions will work fine.
I tried just changing the CD changer unit between these units and they were incompatible with each other. Even the two 1K0 035 180C would not work with each other. The changer portion worked correctly but no sound came out.
I ended up buying another 1K0 035 180C unit, its faceplate is a bit bubbly and the seek button is worn white but it works.
In short CD changers are not interchangeable even when the same model. Just buy whole new unit instead of trying to trade internals.