We have 2 focus's in the family a mk1.5 and mk2. They have both suffered the same problem with brake lights failing to work properly and each time it was discovered to be the bulb holder.

Some moisture got in and corroded the terminals which is seen by a white residue powdery substance. It's easy to remove them for a quick check, hope this helps .

edit: good advise above from Tommy to swap the known good side bulb holder over to test. Should be an easy way to check.