I have to do mine sometime...I assumed it was just clipped and use of a suitable lever would pop it off (possibly accompanied by the sound of shattering plastic ).
The lower circle MIGHT be domountable.. with a threaded collar under it
This was the norm back in the 90's
This retained the bowden cables..
And the trim plate then just hooked onto the doorframe ( the rubber door ..or window strip could cover one edge of it..)
Hi, this part is clipped in place. If you shove a wide blade screwdriver of interior trim removal tool (also known as wide blade screwdriver ?) in the areas arrowed, and lever it off.