Ok, made a bit of progress on the back- it's fiddly but not that hard to be honest. Most of the bits & pieces arrived and for the first time in about a month it was sunny here so I had a stab at getting one side done.
Feeling stupid? Let's go put rivets in an SLK
The goal is to press one of these guys into the hole next to the drain below the C-pillar (rivnut details are in a previous reply):
This is a roof-down, trunk-open job. I didn't bother putting the trunk lid into assembly mode, but for the left-side I will; the working space was pretty tight and much cursing ensued. First, prize off the plastic leaf-guard with a trim tool or flat-bladed screwdriver. Then gently lift the parcel shelf up as far as it'll go with the trunk lid up. Pull the end of the lower trunk lid seal from the top, towards the back of the car:
You'll have to (gently) pull a bit harder when you get as far as the extended flat lump of rubber that goes over the drain hole since it's held on with adhesive. This video https://youtu.be/Q0Z-EVP0NYA
shows the process fully but you only need to remove the seal as far as the plastic side-stopper for the trunk lid. I found 10 years of dirt ingress under the seal when I did that- yikes!
I cleaned the area, first with water + household detergent and then wiped down and dried with isopropyl alcohol. You'll also want to remove the rubber stopper from the hole where the rivet goes:
Now we get destructive- the WIS doc says to bend back the sheet metal lip holding the seal. I don't think there's a clean way of doing this, especially on a curved surface. I used some mole-grips with some microfiber fabric over them to avoid damaging the paint:
Bend the bare minimum necessary for your rivet tool to have a clear working area around the hole.
The yellow residue in the picture is leftover adhesive from the seal, not dirt. I'm going to order an entire new seal having seen the mess underneath, but that's a job for another day.
I already had a rivet-nut gizmo at home- a Sealey short-arm hand tool, which gives more control over force exerted but does require more space than an electric one. Whatever you use, take your time- the flange on the nut should be seated absolutely parallel with the paint surface and these things are a total pain to re-pull. Pic in action:
To reassemble, perform the above steps in reverse. Clean the leaf guard before replacing- it has a bevelled edge which needs to face down:
...and the final result with everything back in place:
Phew, one side down, one to go!