Hi,
It does sound electrical. Often times, corrosion or a loose battery terminal can do what you described. Take a look through this link. It will guide you through the checking/repair process.
https://www.2carpros.com/articles/everything-goes-dead-when-engine-is-cranked
Keep in mind, the battery itself could be the problem. If the above link doesn't help, remove the battery and have it load tested at a parts store. Most will do it free of charge.
If you have to have the battery checked, here are the directions specific to your vehicle. The attached pics correlate with the directions.
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2005 Cadillac STS V6-3.6L VIN 7
Battery Replacement
Vehicle Starting and Charging Battery Service and Repair Procedures Battery Replacement
BATTERY REPLACEMENT
BATTERY REPLACEMENT
REMOVAL PROCEDURE
pic 1
1. Disconnect the battery negative cable (2).
2. Disconnect the battery positive cable (1) from the battery.
pic 2
3. Remove the battery heat shield.
pic 3
4. Remove the battery retainer bolt and battery retainer.
5. Remove the battery from the vehicle.
INSTALLATION PROCEDURE
pic 4
1. Install the battery into the vehicle.
2. Install the battery retainer and bolt.
NOTE: Refer to Fastener Notice in Service Precautions.
Tighten the bolt to 18 N.m (13 lb ft).
pic 5
3. Install the battery heat shield.
pic 6
4. Connect the battery positive cable (1) to the battery.
Tighten the bolt to 17 N.m (13 lb ft).
5. Connect the battery negative cable (2).
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Let me know what you find or if you have other questions.
Take care,
Joe
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Wednesday, December 9th, 2020 AT 7:18 PM