Think you've been given some duff info there Vlad
I've researched this numerous times and spoken to quite a few traffic cops over the years who all come back saying that it has to be on the most front part of the car
Putting it in the window may be making it visible, but is illegal - I think the cops just give a bit of discretion when it comes to that from my experience though
quite possibly I been given duff info... been reseaching myself and couldn't find any def. answers
But as kris sad above - if you dont go over the top with it or do anything stupid - dont think coppers will give you any trouble... but at same time all depends on the copper(s)... as some of them are n*bs!
It's all about attitude really, the new style 'producers' have vehicle rectification notices on them aswell, meaning an officer can issue you a ticket stating you need to get the plate sorted out, then you take it the the garage which stamps the form to say the work has been carried out, you then produce this form at the station. Alot better way of dealing than dishing out fines for what is really a small offence!
But yes, unfortunately some officers (usually traffic) can be a bit anal to say the least.