Absolutely nothing electrical is working. Battery checked and is fine

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Absolutely nothing electrical is working. Battery checked and is fine. Tried cleaning the terminals as there was corrosion, put them back on and it worked. Half an hour later car won't work again. Checked the battery again, it has full charge. The first time I tried turning it on everything flashed and then the car went dead. No lights, radio, dash lights, or ignition. Also if it helps it has a 1996 Infiniti g20 engine in a 1995 body. Almost everything other than the engine and alternator are 1995.