http://www.ftooc.org/?section=faq under this question:
(under question: I want to replace my rear speakers, but I've got a windscreen wiper)
I get stuck at the "removing plastic disc" section. This is a photo of where I'm up to - I've removed the plastic top and undone the bolt:

If I slide up the round plastic thats kind of in the middle layer, I can see a bolt that needs to be undone with a spanner, but its a bit hard to get to.
And I can't see how I can possibly do the same on the front - there's just no room:

Help please!