Join Date: May 2010
Vehicle: 2005 SLK 55; 2005 SLK 350
Other Toys: Several past R170's
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The only thing I'll add to your manual opening ordeal is this:
There are two outboard hinges between the roof and the rear window, and these hinges include a sliding mechanism that "releases" and "seizes" the hinges. If the hinges are not released you won't be able to open the roof manually. It will move back a few centimeters from the windshield but no more.
The sliding mechanisms are operated by two Bowden cables that are attached to the two rotary latches (not the same cables as the boot lid release). A spring counter-balances the mechanism; you can see these mechanisms (especially the springs) clearly by looking up into the hinge areas - even with the headliner in place.
When you turn your Allen key manually to unlatch your roof, you need to turn it slightly past where the latches fully unlatch - to pull the cables and release the hinges. You might need to use 2x the force to cause them to release. You might also want to give the mechanism some lithium grease if you haven't recently.
On your boot lid release cables, my experience has been you can't feel any action -- the only way to know you've released those latches is by actually trying to tip the boot lid backwards. Try doing it with a helper and with the trunk open. Hint: use a wood or plastic shim to keep those latches from re-engaging.
Also my experience: the switches to monitor the roof movement get sticky with age, and stick open and closed -- causing all manner of odd failures. My most recent experience: *both* switches in the roof latches were sticking closed. You could tap on the corner of the roof and it would un-stick for a few cycles, then stick again.