Yes 2-wire systems do matter. You have two filaments in those bulbs - a large (bright one) for brake/turn and a small filament (dim) for tail light. If the large one is getting the (wrong) tail light power it will be so bright you can hardly see the turn signal emitted from the small filament.
So both wires are power, the circuit is completed via the metal parts back through the body to the battery. If the ground is weak (socket to fixture, fixture to body, body to battery) the one light might appear normal until the other light is activated and then the resistance is so great both filaments will be dim.
Mikey
