Why are these lights continuously on and flickering?

To get a meaningful answer to your question It would probably help a lot if you provided make, year and model.

 

Are both doors completely shut? Open and close the passenger door just to be sure. If they are, go into the instrument cluster menu, find the provision for the interior lights to be on when the doors are open, and turn that off. You’ll lose your interior lights when the doors are open, but, that should kill those lights.

I suspect, your BCM (Body Control Module) isn’t happy. If you can disconnect battery power, that might reset the BCM.

Given this is a POS PACCAR product, get used to unexplainable BS with the electrics. These trucks truly are garbage.

 

My KW T680 needed the passenger and driver door to be slammed very very hard.

Those symptoms could be from a door almost closed.

Also those light may have switches to turn then on & off when engine is on or off. Try using the on/off switch built into the lights to turn them off.

Once you start the engine those lights will become brighter because of the increased voltage coming from the alternator, versus the battery voltage when engine is off.

 

My Cascadia has that 'door almost closed' thing, but it toot-toot-toots the horn until I slam it shut.

 

Door ain't shut

 

Check the door switch. Seen them fail on the older B cabs.

 

On these, only the round light is switched at the fixture. Even then, with CAN bus systems, you’re not cutting power to the light; you’re only sending a signal to the BCM you want to turn off the light. It’s a stupid way to control truck electrical systems.

 

Here’s a trick for all y’all That never drove a T-800 that sealed like a tank. Crack your windows before shutting the doors that way you don’t have to slam the #### out of them. Only way we could get them to shut completely back in the day.