I have changed my mind and recommend that you get the harness mentioned by Charlie, from Greg Bender. If you don't know, he is at thisoldtractor.com. I guess that this modification merely lets the existing dash lamp function as a turn indicator when the high beam is off. When the high beam is on, the dash lamp probably does not work as a turn signal indicator. This seems fine to me, but you should clarify how it will work when you contact Greg Bender.
The reasons I am changing my recommendation are that I could not find a prepackaged solution in an internet search, and that I believe from having done the search that many people don't do a good job on their own. If you are comfortable soldering a diode in a wire that will be subject to vibration, then go ahead.
Greg Bender's solution also avoids the problem of where to put the new indicator light or the buzzer.
