I dont know on a focus - but sounds like a bad earth somewhere...

Try in the boot (this is usually the first place you'll get a bad earth on the lighting circuits) find where the rear lights get their earth in the boot (should be a wire or two going to a self-tapping screw in the body) and see if its loose/fell out.

If not then get your torch out and start looking around the engine bay at all the earths bolted to the body.

I've got a dicky earth in the boot of my orion, every now and then my lights go all weird (press the brake pedal and the fog lamp switch lights up - switch indicator on, and the sidelight and indicator light flash alternatively) - wiggle the earth in the boot and it all goes away - till something rolling around in the boot smacks it again!

Hope this helps....