Yeh - dont do it. Its bloody difficult!!
Easiest bet is to use a heatgun - like a paint stripper, and a flat blade screwdriver. But the glue they are held toegther with is like tar, and it is really manky stuff.... it is not something i'd like to do again in a hurry.
personally i woulndnt use a screwdriver, patientence and a strong grip is all thats needed
its takes a bit longer than you think to open them up , use a medium heat and dont leave the heat gun on a single part for to long or it will distort the lens or plastic
Patience... that might be the best bit of advise yet.... i was impatient, and tried to use a screwdriver to pry them open. Paul, what do you do with the black goo... scrape it out or leave it where it is?
That black goo is epoxy resin...thats what holds the seal tight and stops the moisture from getting in...I would recommend keeping as much of it in there as possible!
I just tried to open my headlights and failed miserably on the first step...they dont even fit in my oven ... so I guess the only way I can do this is buy getting a heat gun ?