The dimmer switch deals with two different circuits, you are likely trying to combine them.
The dimmer switch, when turned all the way, creates the ground circuit for the dome light. Like the door switch does.
I don't recall which pin is for the dome, but you need the pin that has continuity to -ground- [the metal plate on the switch bezel] when it is fully turned. Your new switch will tie that pin to -ground- to activate the dome separately.
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