Peugeot 106 sidelights coming on when the brake is pressed

Sounds like a bad earth. unplug the connectors on back of both rear lights and clean terminals with sand paper or matchbox strike paper. re assemble and check connection. if still no luck check earth straps around boot area, These sometimes get dirty and/or loose. check clean and tight(Don't apply grease or oils as this will not allow a clear contact between the earth wire and the car body). Earth straps are usually brown and can be single or a few clustered together (all same color)

maybe someone has put a wrong bulb in the back lights