Seriously you have got a Berlingo aint you?
Near the rear doors on the support that runs the width of the roof there are 2 large slots (1 on left side and 1 on the right). The slots are probably 2" wide.
I drilled 2 holes close to the left handed slot within a distance that I knew I could get the bolts through by hand.
I fixed a soil pipe bracket (from wickes) with 2 x 8mm bolts as described.
Then I got a length of soil pipe which slides into the bracket and the other end goes and wedges into the shelf at the top of the windscreen in the cabin end.
Also the pipe is easily removable (just slides out) if you need it to be.
http://www.wickes.co.uk/invt/431865/?source=123_75http://www.wickes.co.uk/invt/431920^
These are not the brackets I used but are the same style. For some reason I can't find them on the wickes site.
You will obviously need a soil pipe bracket the same size as the soil pipe you are using.