I have an odd issue with the girlfriend's car, when you don't have any lights on the brake lights work as they should, but when you turn the lights on (low, main or full) the brake lights come on and stay on, if you press the brake in this state there is a visible flicker, indicating the switch for the brake light is still functioning, but the lights stay on at about the same brightness, this is a rear end accident waiting to happen!

I would assume that this is a short circuit somewhere but am not sure where to look, I've checked all around the rear lights but everything looks fine there.

Any suggestions on what to try or where to look?, or should I just get it to a garage asap?