Engine starts with jump cuts off when jumper cables removed? Alternator or switc

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Had intermittent power loss condition. Replaced battery, ran good for 3 weeks. It shut off on traffic Sunday on the way home from church, lost power to gauges, didn't want to come out of park. R&r main relay, but it shuts off after removing jumper cables. Is it the alternator or the switch. How do I test to determine which one it is?