The front headlights take separate feeds to each main and to each dip beam from the engine fusebox so hard to guess how this could be connected with the brake lights - worth checking these bulbs have not blown. We have a 2003 1.4 petrol C3 and the bulbs that seem to go a lot are the no. plate and left front dip beam. Is everything else electrical working and all the other lights ?
Maybe one last resort could be to disconnect/reconnect the battery in case of a management system error (BSI). If you do, switch off everything, remove the key, open the bonnet and then wait several minutes for everything to shutdown. Then disconnect battery, wait a few mins, reconnect, wait a few mins, put key in ignition, wait a few mins, turn key to first position, wait, turn to light up dash, wait, start engine, reset clock, wind windows down and up to reset anti pinch. Then, if lucky, the problem may have gone.