No power-replaced battery, altenator, and battery harness

I have a 2008 Jeep Grand Cherokee- V6, 3.7L. Jeep would not turn over had
message of Transmission over temp. Not possible with not being driven for
24 hours in 20 degree weather. No power. Mechanic says dead battery.
Replaced battery. 48 hours later, battery light comes on and not getting
enough power-lights dim. Replace alternator. Same day, 3 miles down the
road, loses all power, no lights, nothing- vehicle is dead.
Put in a new battery harness.
Car starts, but wont hold steady charge. Fog lights will not come on, and
seems to have issue still with turning over.
Battery light on still.
Any ideas what is causing this?