having problems with speakers on my 07 silverado classic. right side mid stopped working, tweets fade in and out, rear speakers work when they want to. pulled head unit out and checked wiring harness and connecting pins and seemed ok. i've seen in other posts that this is a common problem for this and other trucks. any suggestions? i would also like to eventually change the head unit to something more up to date, but have heard they are a nightmare to change due to harness is wired into more than just the stereo. please help.