Pat Dorans mk6 rallycross fezzer has a rear mount, if had bought the last few issues of fast ford you would have read all about it
theirs actually runs COOLER than the front mounted rad!
it needs to be a total sealed unit, Pats had 4 naca ducts feeding it (2 from roof, 1 from each rear quarter) and the boot lid chopped out (it was mounted verticle, not horizontal

)
get onto the back issues