On your bike, the turn signal wire colors are as follows:
Rt. Turn Signal: light blue
Rt. Marker: light blue with white stripe
Lt. Turn Signal: light orange
Lt. Marker: light orange with white stripe
Only the front signals are lit (marker lights) all the time. The same wires that control the front turn signals control the back ones. As you have discovered, the turn signal wires run through a different connector as opposed to the wires for the marker lights.
I have no idea whether the problem with your horn is related; however, the (ground) wire for the horn (light green) does run through the same connector block with the turn signal wires.
Does the rear rt. turn signal work? How about the indicator light on the center console?
I created a color wiring schematic for the '77 CB750k. You can find it here:
http://forums.sohc4.net/index.php?topic=33638.0