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rear mounted on a towbar is what I settled on, tried one on the roof but it's just too high.
Somedays you're the pigeon, Somedays you're the statue.
I got a high rear Halfords one of ebay for a few penny's.
Does the job as long as you don't reverse with the front wipers on.....otherwise...well, not sure as not fell for it yet but it looks like it may not be good!