Hi, I am having a Kenwood 5240BT double din fitted on Monday in my Leon FR, the gent fitting it has told me the bits I am going to need are as follows -
Patch lead to suit the stereo; apparantly this converts the Seat wiring to the radio you are fitting, and it also allows the use of the steering wheel controls. You can also buy a standard conversion lead off of ebay instead for about £5 but it will not control the steering wheel controls.
You will probably need an ariel converter to fit the standard ariel into the back of the unit.
When I am down the garage on Monday I can find out the exact bits you need if you want... If so let me know the model of your car and the year and I will ask; I know they do mail order.
Hope this helps!
Simon