...and amir spake unto the tribes of Usenet, saying...

Can't speak for the Disco II, but I've just replaced the one on my Disco I
and it wasn't too bad a job to do.  The six bolts screw into nuts on the
inside which are not held in any way, so they will fall down the inside.
You have to remove the inside panel (Austin's advice on fasteners is sound,
although I got away with it) and if you have speakers in the door you will
may to remove them, but no big deal.  On the Disco I some of the nuts are a
bit hard to get to and I ended up using various bits of wire etc to hold
them as I put the bolts in from the outside.  An assistant would have been
very useful, but she was busy.  The plastic sheet inside is held on with
sealant and you will probably rip it, so have some sealant handy (I used
clear bathroom sealant) to fix it back in place and make sure it's shedding
any water that gets inside in the right direction.  If you work carefully,
both getting the panel off and putting it back on (wallop with heel of hand
is the best way to get the fasteners back in) and you won't know it's been
off.