Take a look at your tail lights. You will have a ground and two other wires on each side, correct? (I'm assuming that you've got a single bulb in a rectangular light.) It that's the case, there's a relay somewhere that takes two signals (one from the brake light switch and one from the signal switch) and translates it to a single signal to the brake/signal filament at the rear light.
The early Contemporarys may have used a Lucas relay to do the translating.
It may look like like the silver box on the firewall in the foreground.
