99 Intrepid - no electrical power

My wife's 99 Intrepid was running fine one day; but, the next morning it would only "click" when she tried to start it--and finally did NOTHING!
I replaced the battery after it tested weak at the local auto parts store. But, the car still won't start, and there is NO power to the interior systems. I tested the "remote" jump-start terminals and get just over 12.5 volts, and I get 12.5 volts at the input to the fuse-panel housing.
Anyone have and idea what the problem might be, or what I might check next? Thanks for any suggestions!
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