I have a vauxhall Astra. When I switch my side lights on, my break lights come on and stay on. when my lights are off the break lights work normally

Phil, August 2008

Earth fault. Look for damaged/broken or disconnected earth wire for the rear lights.
Your side lights are using the positive side of the brake light circuit to work. If you press the brake pedal (ign. on) you will find that neither will work or will go very dim.
Prove it by running a temporary earth from both rear lights to chassis

Cledlyn, August 2008