Def recommend cleaning the contacts and checking the fader/balance controls. Also, try a different CD if you havent already. If all else fails, and your head unit is in fact, bad, I have a 6 month old refurbed one for available for sale.
The CDR-220 was the latest non-MOST bus CD player, wich changer input and aux (iPod) input. Let me know if you're in need of one as I just pulled my recently refurbed one out.