Firstly, have you checked it isn't already working? I'm wondering if (guessing) it might use the same red-coloured sensor on the top of the dash that the climate control uses.
I know this sounds mad, but on some previous model SEATs and VW/Audis you could add cruise control by fitting the stalk that had the buttons. Everything else was built in as standard, you just needed to add the button to tell it to do it.
Edit: Another thought - if you had a photo of the scrapped car, did it have the automatic wipers? I'm wondering if it was built into the mirror unit that contains the sensor for the wipers. You can see a black blob on the windscreen where the rear view mirror is.