No start, no cab power issue

[Engineman2018]

Good morning all,

Truck is a 2019 petervilt 389 with an X15.

I have an interesting issue. A truck i work on broke down at the local coop. Driver stated "truck wont start, started many times previously this morning," boss suspected a starter. Got to the coop and tested battery voltage and connectors, voltage at starter and voltage while cranking. All wired and cables checked out good. I noted that all voltages were good but there was no voltage from the exciter wire. I turned my attention to the cab and fuses and immediately noticed there was zero cab power. No cab lights, no dash lights, no headlights, clearance lights, brake lights, turn signals and hazards. no radio or CB chatter. Wipers worked on high but no other settings, washer pump didnt work, no light switches worked. While keyed on, the ECM did wake up and THE ABS module went through start up. I checked all fuses, powers and grounds and relays with no discrepancies found. With the coop backing up, I decided I'd see if the truck would start. I keyed on and jumped the starter and moved the truck out of the line and to a better area to work on. I inspected a few more things before we got rained out and moved the truck to the shop. I removed the grounds from the battery while we discussed a plan. When I reconnected the grounds, the issue was no longer present. All cab power returned and everything worked as should. I cant wrap my head around it and cant replicate the issue. Any insight would be greatly appreciated. Please let me know if you have questions or need clarification and ill get it done.