2002 Rendezvous Electrical/computer issue

I have a 2002 Rendezvous and all the gauges, lights, radio, flash on and off, click, tick, doors unlock, lock, you name it, it all goes awry. I was told the computer module was shot. I ordered a certified tested used one from Cherry Automotive. Changed them out, the same thing is happening with this one. I'm thinking that it is NOT the module and there is something else causing this to happen. Any suggestions as to what might be the problem? Thanks