Looking at the 81SC schematics both BL/WT wires from the tach directional lights are tied together to the C terminal on the flasher relay connector. There is no way to disconnect the BL/WH wire run without doing damage and or permanently modifying the wiring or connector. Again, the simplest mod is what I have done and it is completely reversible. I tested my circuit with a three pronged flasher I bought at Autozone. Since I knew that my Hella flasher relay failed and was supplying stray voltage onto the terminal C causing the two tach directional lights to light continuously. I decided to open the flasher and unsoldered the terminal C prong and remove it from the flasher. This removed the stray voltage from the tach directional light circuit. When I tested the flasher relay in the modified circuit it worked and the two tach directional lights operated normally.
This is the original Flasher relay that had failed and was supplying a stray voltage instead of supplying a ground to terminal C.
This is the relay with the C terminal completely removed. (now it is equivalent to a Three pronged relay with terminals 49,49a and 31).
This is the relay connector with a male spaded ground wire plugged into the C terminal.
This is the modified flasher with the modified flasher relay connector.