Hi,if i remember right the MD of the garage i worked for orderd one of these for his mother inlaw,we painted it booked it in...and ...well if i remember right there's a brovo rear screen that comes with bolt holes (it went back into parts!!)
Their used to be a kit for fitting mk1 punto ones which came with a glass primer, if i remember right ur spoiler fixing holes are all over the glass area???,the kit was about £30 but its a while ago,but even then the odd one came a drift,the answer was scratch the glass up with some 80's sandpaper,glass prime then a super strong bond,should be able to get some from a windsreen company im sure!!
but try leave it till the weather warms up else it shart just be the rear wiper u c moving across the window.
hope this helps,if u struggle to get some window bond/sealer give me a shout and i will try get you some cheap.
Regards
Richie