The dome above the instrument cluster.
Its been ages since I done this, but from what I remember you take off the steering cowl, the 2 screws at the bottom of the clocks(cant remember if you have to take off the panel below the steering column for these), then the whole lot pulls out which can be tight as there is a strong clip at the back of the dome. I never went any further than that as I was only fitting a speedo converter. Hope that helps but reading back it probably won\'t.
Phil