150 FHC interior light

The light below the rear window wont come off with doors closed. I have had the bulb out for a couple of reasons, for this reason. I know that I fiddled with it in the past. Yesterday I inserted the bulb and it worked with the door open, and went off when I shut the door, but not for long…
Today I thought I was lucky when it went off when I turned on the headlights, or already with the parking lights, but then came on again.
Later it would shut off with the lights in the Fog position, only to come on after a few seconds.
What do the electricians suggest?
(The contact in the door with its purple wire appear very normal)

The door contacts are probably oxidized.
A good clean would cure that.
You can reach them by removing the side panel at the A-pillar

Mine has a switch on the dashboard which also operates the light. Just wondering if you might have pressed that ?

Very good point! Yes, I have the INT switch, and it turns the square switch part 90 deg on each press of the button, but the light remainsnon, unfortubately.
Tomorrow I will chech for corrosion inside the A pullars, and if not successful, I will remove the cover under the instrument board and investigate

I notice how the power of the interior lamp drops about 10% when I push the switch in the A-pillar. Does this point to the oxidation theory (at some sort of contact behind the panels agaibdt tge A-pillars)?

Yes, oxidation in the door switch seems the most likely cause.

Took the contacts out of the A-posts, and they look fine.
So ground is touching the purple wire somewhere between the A-posts and behind the dashboard…
Or on the way back to the lamp under the rear window.
Where is the cable located?