I think all Fulvia coupes had provision for speakers in the doors (the TAV shows the speaker cut out), as a radio was a factory option from the S1 onwards. My only problem was finding a speaker of a small enough diameter that would fit behind the original speaker grills - try finding anything under 500 watts and smaller than a dinner plate ! It didn't have the correct PCD so I made up an interface ring so as not to drill the doors.
The aux input with the Becker is quite forward thinking for 1969 - they modularised their in-car systems so you bought the head unit but could add cassete and 8 track players. So using this input means I can plug anything into it now and it plays through the amp and the speakers, MP3, Sat Nav, hands free phone etc with no modification to the stereo or the car and have it all hidden away. Quite chuffed with it really