there should only be one wire to the bright filament [brake], the brake lights go from the stop light switch to the signal unit, and when you select a direction to flash, that brake light is used for the signal, the other brake light works normally ....
here's a factory diagram for the 55-59, should give you the basic 'map' to get your lights wired right, the 2 brake lights must have individual wires that route thru the signal switch, yours are OK for no signals ["chained"], but won't work with signals
edit: remembered I have a
good aftermarket diagram here. that may be better help
Bill