School me on interior dome/courtesy light circuits... (1 door = no illuminatin')

Jun 24, 2018
East Bay Area, CA

Title's self explanatory... And I ***think*** I've isolated the cause of this issue, but need confirmation from someone wiser with chassis electrical than I am.
Here's the related electrical diagram for the chassis section I'm working with:
Related Diagram.png

Dome / "Courtesy" light illuminates for 3 of the 4 doors.
FR Driver, FR Passenger, and R Driver all illuminates the dome lamp... but the R Passenger does not illuminate.

I replaced the switch with an OEM part, and still have no dome/courtesy trigger when I open the door.
Sanded the chassis contacts.

Questions I need answered:
1) Does the RED-BLUE wire in the chassis dome light circuit function like the Horn circuit, in that a grounding triggers the horn?
2) If so, would I see a 12v current at the RED-BLUE chassis wire at all times?

See edited diagram below for visual explanation:
Issue Explained.png

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