ok guys heres the deal, get the car back from dealer cd changer has everything plugged in but when i go to switch to cd it doesnt show up as an option (i have nav)

problem 2 my sound keeps jumping one minute it sounds full of life the next it sounds dull, ive had my amp changed 2ce, my headunit changed once... it sounds almost like some speakers get disconnected or just loose connection and the sound goes from being liek "surround" to like a dull sound any ideas? the dealer claims its the amp but i think theres something more to that