Radio help please


I've just replaced the radio in my 2007 Sirion with a radio from a 2009 car. I had a volume control fault so took the opportunity to install a newer unit with an AUX input.

I have an AUX cable running into the back of the new head unit, but I cannot for the life of me figure out how to select it as a source!

The buttons on the front are identical to the old unit, the only difference I can see is the additional port on the back where the cable goes.

How do you select AUX as an input? Any help appreciated!