Well I tried to find out whether there was a DIY for replacing those two little flaps (hinge covers) on either side of the parcel shelf, but to no avail, so I figured I'd post my first solution - quite easy actually.
First of all, I used another thread to remove the parcel shelf itself (linked to roll bar removal) at:ForMyMercedes.com
. I'm hoping that this isn't consider infringement of any sort.
While I didn't complete the entire process to get to the covers, I found it very useful and simple to follow.
Step 1 - remove the mushroom caps below each rollbar
Step 2 - Locate the three screws below the top of each seat belt [where the seat belt enters the body of the door frame], unscrew using a Torx screwdriver
Step 3 - Carefully pull the metal plate out in a downward motion
Step 4 - Inside the seat belt assembly [where it enters the door frame] you will see a Philips head screw towards the inside of the car. Remove carefully with magnetic screwdriver so it doesn't fall inside the body forever. This screw hold the plastic upper seat belt cover to the centre parcel shelf
Step 5 - Once removed, move to the hinge cover flap and lift it up [towards the outside of the car]. You will see a single bolt - remove with an 8mm wrench - again, be careful you don't drop it. Take it out by hand if possible, once loose.
Step 6 - Carefully work the plastic cover piece free and let it slide down the seat belt - it should be fine there
Step 7 - Now you will see an odd shaped metal hook at the front edge of where the plastic cover was [towards the right side of the flap] This is the location where the strap should be attached with its integral rubber loop [mine where broken off and only attached at the back under the mechanical portion of the shelf] - don't attach the new hinge cover yet - it's just to know where it will go later
Step 8 - Now you have to get the roof into the trunk (almost), in order to allow the triangular-shaped corners to lift up in a semi-permanent position [ I had to leave the ignition on, so it didn't close up on me
Step 9 - once they are in the vertical position, you can see where the covers actually connect ] two screws hold a small metal bracket which slides into the end of the hinge cover
Step 10 - carefully unscrew the two bolts [making sure they don't fall down again], and remove the metal bracket with the remnant of the hinge cover. There are three Torx screws there, but only remove the outer two
Step 11 - once removed carefully slide the bracket out from the old hinge cover, noting the orientation, and insert it into the new cover
Step 12 - replace the bracket and reinsert / tighten the two screws
Step 13 - slide the other end of the cover under the triangular-shaped corner to get it out of the way
Note: at this point, my car started to beep at me [probably cause the roof was finished], so I turned the ignition off, let the roof settle and put it back again so I could do the other side by repeating steps 2 -13
Step 14 - once both side are completed, and the two hinge covers are facing forward, you can turn off the ignition and let the roof and parcel shelf settle again, but don't reopen / reclose the roof yet.
Step 15 - Now attach the two loops from the hinge covers to the odd bent metal arm forward of the hinges
Step 15 - Now reinstall the seat belt cover, not forgetting the bolt under the new hinge covers [probably a little stiffer now :-)]. You may have to realign the threaded part of the plastic cover for the bolt - it slides laterally. Again, be careful not to drop the bolt / screw on reassembly. Reinstallation is just the reverse of disassembly as always. Hope this helps someone else out.