[Blas]

I believe the box you are referring to allows the duel filament, single bulb rear taillight to also function as a turn signal when your foot is on the brake pedal. Consider the senerio, rear taillights, left side, one bulb, low and bright filaments; headlights on, stopped at a stop light, car rolling down hill so foot on brake, and making a left turn... The bright filament needs to be on for the brake light and yet still flash for the turn signal on the left side.
I have Wiring Diagrams that I just finished for CSX chassis that use a similar system. And the new Superformance MKII Slabsides chassis also uses this system. Both of their wiring diagrams (CSX and Slabside) are similar in layout. Drop me an offline email and I'll give you my phone number. We'll get you blinking in no time.