[HummerNewbie]

I you want to find out if it actually is the Nav unit, unhook it and see if your battery goes dead. Do you have to manually turn the unit off whenever you turn the vehicle off? If it goes off by itself then it actually is switched and the Nav probably isn't the issue. Could be somewhere else in the install that is causing the problem though. From what you have said, sounds to me like you are in need of a new shop that knows what they are doing. Could just be your interpretation of what they are telling you but sounds to me like your shop took short cuts they shouldn't have.