Malfunctioning passenger side rear turn signal & brake lights

I recently acquired a 1973 Cadillac Coupe de Ville back in July, 2023.

PROBLEM:
When I took possession of the vehicle, all of the exterior lights (head lights,
front signals, rear running lights, rear brake lights, and rear signal lights) were
functioning perfectly. The interior indicator (for lights on, braking, and rear turn
signals) just above the rear bench, above the rear window, and mounted at the
rear of the headliner were functioning perfectly. Out of nowhere, just a few
days ago, I had a friendly driver pull up next to me and inform me that my
passenger side brake and turn signal lights were not working. The rear running
lights were still working on both sides. I then noticed the passenger side
internal indicators viewable from rear view mirror were not lighting up when
braking or activating the passenger side turn signal.

QUESTION / HELP NEEDED:
I'm new to working on cars (new/old) on my own, but am mechanically savvy.
How do I go about diagnosing what might be causing the above problems?
What are some of the reasons any of you have seen this happen?

NOTE: I have the original "Cadillac 1973 Shop Manual", but I'm not able to get
the guidance I need from it. I was going to just try to replace the bulb, but the
manual is telling me that in order to get to the bulb, I need to remove the rear
bumper in order to then remove the tale light housing. As I'm sure you can
understand, I don't want to go down this road if the bulb is not the problem