Hi , as Jay says , dropping the headlining is not easy to do without damage .
However , if you are just trying to fix a leaking drain ( they rust where the pipes exit from the corners of the cassettes ) you may be able to gain enough access just by dropping the edge of the headlining at the top of a door ( I did this on my 190E and made a repair by cutting away the rust and using fibreglass - not an ideal repair but lasted as long as I had the car ) . To do this it was neccessary to pull down the door seal from the top , and remove the grab handle from above the door .
It has been so long since I had a W123 apart that I can't remember clearly , but on my W126 , once the rear roof and C post trims are removed , the back end of the headlining can be removed without difficulty , but it does not go all the way to the window on that model .
Removing the sunroof door is easy - slide the roof back and the headlining of the sliding panel can be popped down at the front edge and slid out in a forwards direction . If you have an estate with the manual as opposed to electric sunroof then you first will need to remove the trim around the handle . With the headlining out of the way , close the roof again and you will see the bolts holding the panel to the mechanism - unbolt and remove .
If yours is a particularly good W123 , or you plan to restore and keep it for a while , you might want to consider a professional repair - Mark Cosovich , a W123 specialist , normally keeps a couple of refurbished cassettes ( rust cut out , new drains welded in and repainted ) in stock and can either replace yours for you , or will supply a repaired cassette on an exchange basis against your old one . Mark normally advises removal of the rear window to get the cassette out of the car , although I think it should be possible if you take all the seats out to get room to manoeuvre inside the car and bring it out of one of the back doors .
