Depending what year your car is, an therefore the aerial connectors you can do what I did which was to just remove the white and biege aerial caps (they just unclip) and push the now bare plugs onto the back of the stereo. I have a link howling what I mean, will try dig it up for you- saves buying and adaptor and works perfectly
Editcouldn't find the link, but found this picture - which although from the US shows the connectors. I had these, just took the plastic covers off so i can plug both the connectors in the back (as too tight with the plastic on which im sure you know), therefore not needing the adaptor
