Start battery voltage to rear, with out ACC on

[the evil twin]

Power for the dome light comes from the A pillar along roof then to light. Tipping it would be pretty tricky to try and route a wire from this down the D pillar and that would be with pulling back headlining and dicking around with it. Might work but think would be much easier to run a new one along chassis or body in the floor IMO.

There will be power to the courtesy switch on the rear doors, that would closest i think unless there is one to do with trailer wiring.

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Pretty much agree... not sure how easy/hard it would be to get a small gauge wire from the dome light but is only option I can think of if OP wants no seperate feed.

If I can picture the lamp wiring cct correctly (so may be worng here) the Courtesy switch feed won't be ideal as it is downstream of the lamp so any time that any of the door switches are made the voltage will be zero (ish).
Same if the courtesy lamp is 'isolated'