that's not really your concern is it?
if it someone can't tell the difference between fogs and brake lights.
I've had twin fogs on my bmw and got them on my A4 with out any issues from other road users or MOT testers.
can't you route the cable through the weather proofing to save going out side or cutting?
It's not something I'd worry about either, in the 5 years I've had the car I don't think I've ever used the rear fog light.
The issue isn't so much people mistaking them as brake lights, but when you've got them on their brightness masks the effect of the brake lights so people may not see you stopping. Admittedly with a high level brake light the chances of this happening are reduced.