Have you cleaned the contacts on the plugs at the back? They are the weak link because they don't contact very well. You will probably need to bend them a bit to make a tighter contact. When you unplug them you will see what you need to do. They need to be very clean on both the plug and the pins on the lights. I had a lot of trouble with them on my 1988 1.5. If I had kept the car longer I would probably have soldered wires onto the pins. Our current 1.7 is working well with a clean up (touch wood). I was always worried about the visibility of the feeble single brake light on each side so, on this car I wired the two inside red lights, which are probably fog lights in Europe, as brake lights, giving me four brake lights, which is a big improvement. You also need to clean up the earths and give the lights a good clean out with soapy water to improve the brightness.
2004 GT 3.2 V6 (Stromboli Grey).
2008 159 TI Sportwagon 2.4 JTDM (Stromboli Grey).
1990 75 3 litre Potenziata (black), now for sale.
1990 75 3 litre Potenziata (grey, sadly deceased due to fire).
1988 75 3 litre (red).
2000 156 2 litre Twin Spark, ( Cosmos Blu metallic), daughter's new daily driver.