Front side lights come on when braking

[focusdriver]

On my mk1 focus the front side lights come on each time I brake. This has happened ever since I replaced the passenger brake light. Apart from looking silly to oncoming drivers everytime I brake and the risk of giving out wrong and unintended signals, would this cause my car to fail an mot?

[focusdriver]

Oh and is there a fix for this!

[focusdriver]

Thanks, I will double check the bulb and clean the connections.

[focusdriver]

Mr2Mike said:

jagracer said:

You've put a twin pole bulb in a single pole socket or vice versa.

Only one of these combination would cause a problem (single filament bulb in twin filament socket).

Turns out that I put a single pole bulb in a twin pole socket. Replaced with twin filament bulb and all OK. The brake lights were also permanently on when I switched on the headlights which I hadn't noticed before. I can't believe having a wrong bulb in would cause other lights to come on. Thanks everyone.