I don't have a 750, my cx500 and s90 are wired to have separate sub harness for the tail lights, the 550 is probably more similar, on second thought. The wires leading to the tail lamp and turn signals, check for power on their bullet connectors where they plug into the harness. The ground circuit for the bulb is the metal body of the signal and where it bolts in. The two wires are either constant on when bike is on or on when flashing.
Fuses are good and your tail lamp is it lit? Take it your rear markers are dual circuit and normally lighted from your initial post.
You can use a sewing needle to pierce the insulation if necessary to check power along a wire but, generally best to use the bullet connectors when possible.