Aftermarket Radio Will Not Work GMC Yukon XL?

I have a 2001 GMC Yukon XL that I have tried multiple times to put in an aftermarket stereo. I tried the cheap connector that didn't allow OnStar to still work. That failed miserably, the radio would work intermittently, it would power on, you just didn't get any sound from it. I purchased the correct one that Scoshe recommends and tried it, same issue. After talking to Scosche they had no idea, recommended I take it to the dealer. The dealer checked it all over, and they couldn't find an issue. Any idea what might be going on here. Overall issue, radio powers on, reads CD, but no sound comes out most of the time, occasionally radio will work just fine, but may turn off for no reason. And I have tried 6 different head units, Pioneer, Sony, Alpine, Dual, Kenwood and others.