2013 Side Window Latch Missing Seal Washer

[csrote]

2013 Berlingo Multispace
Side Window Latch Missing Seal Washer. UPDATED with parts source
Prevents window from closing tightly.   Not water-tight for car wash.  Wind noise at highway speeds.
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Can anyone provide explicit instructions for disassembly and reassembly?
I have Torx tools and am generally handy but am not sure I see how to access / remove Parts 71 and 74.
Please see parts diagram and photos.
Thnx for all help!

Parts_Diagram
Black_Ring_is_Seal_Washer
Missing_Seal_Washer
Window_Latch_Cover_Fixing

Proposed Source of parts:
Genuine Peugeot Citroen DS SEAL WASHER 698807 Shipped Worldwide (peugeotcitroendsparts.com)
Genuine Peugeot Citroen DS CAP NUT 857379 Shipped Worldwide (peugeotcitroendsparts.com)

[cancunia]

The only suggestion that I have is don't try this alone!
I'm basing that on experience from trying to take out the rear window on my M59, my arms were not long enough to reach inside & outside at the same time.

[brodfather11]

If you remove the screws holding the latch assy. to the body you will get enough room to get your hand in from the outside to undo the inner screw and hold the outer cap yourself.

[csrote]

If you remove the screws holding the latch assy. to the body you will get enough room to get your hand in from the outside to undo the inner screw and hold the outer cap yourself.

Thnx for this advice.
If you look at the last image I posted above it shows what I believe to be part #74. 
This is a cap that must be removed to access the inner screw.
Can you please advise on how to remove this cap, especially without scratching it or anything else?
Thank you.

[brodfather11]

IIRC the cap is just pressed on and pops off without too much bother with a small screwdriver or similar

[csrote]

IIRC the cap is just pressed on and pops off without too much bother with a small screwdriver or similar

Can't get the cap off.  Aargh!
Will think about drilling the cap to remove it...

The other sliding door has no problem with its window latch.
Its cap is even slightly sunk below its surrounding surface so it would be even more difficult to remove.