There's a bulletin located here...maybe this will help...
In looking at my Clymer's diagram, there's no mention of black or white wires for the turn signals. A single wire blue-black goes out to the right signal; another single wire blue-red goes to the left signal. The ground for the circuit is out past the light. The power flows through the single wire, through the bulb, and then to ground.
These same colors go back to the handlebar switch for left and right. There's a small black wire in the headlight housing which attaches to the neutral light. Another black wire attaches to the "HO" terminal on the circuit board...this appears to be power for the horn. I see a white wire going from the light/dimmer switch to the headlight bulb in the housing. There may be other black/white wires that attach to the circuit board but the diagram isn't that specific. You should find a color coded diagram of the wiring and trace everything out.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2