Personally i would upgrade the entire system, i.e. all speakers & headunit. But if you really want to diagnose it, i would start with the speaker wires & inspection of the speakers themselves. Ther're probably blown or have a loose wire. I'm going to replace mine when i get back to the states. If that isn't you're problem, then it is probably the headunit. You gotta think, how many people have mashed the crap out of the buttons or just used them thousands of times before you? Hope this give you a start! p.s. also check the wires going into the amps to make sure they are plugged in all the way and are not worn/ grounding out..