2001 Citroen Xsara Picasso brake lights

Hi,it does sound like a switch problem. It is located at the very top end of the brake pedal. This is a common fault and it is usually the microswitch sticking. If you run your hand from the footpad up the front of the brake pedal metal arm to the very top, up behind the lower dash area bit, you will find a small plastic box with a button protruding. The arm sticks against it and depresses it in rest mode leaving the lights off. As the pedal is depressed forward, the button springs out and the lights come on. It is very hard to get it off and replace due to access, but cheap if you can get at it. I just pushed the pedal in and then pushed and manipulated the button to free it in and out and stop it sticking and its been fine for ages now, the problem never came back, so try that first before trying a new one.