Here is a picture of the signal wiring from my '81 cb650c for reference:
I know you think you are past a wiring issue, and you may be, but just to be sure, please verify the following information.
There are 2 blue (and 2 orange) wires, one with a white stripe, one without. The one with the white stripe connects to the black wire with 2 bands on it (one blue one white, or one orange, one white). The wire without the stripe connects to the the black wire with the single band (either blue or orange, respectively) and to the double blue (and double orange) wire. The one(s) with the stripe connect to the front signals, on the running light filament of the bulb. The one(s) without the stripe connect to the front and rear signal filaments.
You are correct, the bulbs are grounded in the signal housing, which is grounded through spade connector in the headlight bucket, mentioned above. The bucket, and turn signal ears are isolated from chassis ground by the rubber insulators that mount them to the forks which is why this ground wire is needed.
Here is a good color wiring diagram if you want it:
http://i.imgur.com/r9U2y8H.jpg
