changing aerial on ford cmax?

remove the interior light to get access to the aerial.

Most roof-mount aerials don't use a base nut for attachment as wing-mount aerials do, but a screw that's accessible from outside of the vehicle. Sometimes you have to carefully prise the aerial housing up to gain access to it, whilst others are more or less plainly visible or under a small plastic cap that can be prised out with a narrow blade screw driver or sharp pointed tool.