I know that the American Foci rear light assemblies are different to ours. From what i can work out:
The rear tail light assembly has three bulbs in it. The bottom one is white, used for reversing. The red top section of the tail light has two bulbs, and is the combined flasher/brake light. When the car is braking and indicating, one light is steady red and the other is flashing to indicate.
I think lime4250 wants to convert one of the two bulbs in the red section to amber, and re-wire it to work as a flasher only. My only concerns would be that the red lense would make the amber bulb still look red, and therefore the car would just look like it is braking, and not indicating also.
It could work, but that is my concern. Hope what I said makes sense.