I have a mondeo and fit them to the top of the mirror housing. I tried the bottom but the Milenco hit the door when the car mirrors retracted on locking.
I have an insignia, and have fitted Milenco aeros to the top edge of the mirror - tight fit though! - I also use suck it and see mirrors whicha are a lot easier to fit
We have a pair of Milenco Aero Flat Mirrors and a Vauxhall Zafira. They will fit on the top or bottom of the mirror, but I have found that they are much more stable when fitted to the bottom of the mirror.
The easiest way to fit the Milenco towing mirrors is on the straightest contour in order to put less strain from the two clamps on the plastic door mirror housing, so if the top is more curved than the bottom then the best fit should be on the bottom edge with the extension bar going out and upwards. That is the way I would fit them anyway for reasons mentioned. I have the Milenco Grand Aero mirrors but the countour on my door mirrors are straight both top and bottom so I automatically fit mine at the top.