Directional signal question

[GB SISSON]

I bought a sort of generic china looking directional switch at the Portland swap meet last year and tonight I installed it (mostly)... I have it with a 3 prong flasher and a 15 amp fuse all to the yellow sheet of instructions. The fronts ( I had installed 2 prong parking lights in the fenders, are blinking like a champ. Very bright)I have wires leading aft to the new repro tail lights which have worked perfectly for two years as brake and tail. Well to get my rear tail lights to perform in the normal manner the directions tell me to hook their red wire to the cold side of the brake light switch . Do I disconnect the wire that is already there? To me the brake light circuit always energizes both sides at once while the blinkers obviously work individually . Just hate to screw up a good setup with the poorly worded chinese instructions. I think I am 90 % there with this hookup but need more ifo on this back half. I think if I just put another set of lights on the bumper for blinkers it is straight forward, but entering the brake light switch perplexes me. Thoughts/advice? Thanks. Wanna run this truck with my big flag into town tomorrow, my wife along piloting the jailbar woodie!