There has been at least one excellent posting on fitting turn signal audible warnings, using a single buzzer and a diode to keep the two circuits separate. I thought I would add my alternate version (with pictures of the process on my GTS) that uses two buzzers so here goes:
1. Buy two 12v buzzers from your local radio parts store. Mine cost £2.70 for two (less than $5 US) and I chose those with the lowest pitch of the selection available.
2. Remove the front turn signal units (1 screw each side on GTS) the horn casing (1 screw behind flip off Piaggio badge) and the black cover at the left side of the bulkhead (1 screw).
3. On each side, disconnect the turn signal light unit and bare each of the two wires connected to the white plug.
4. Solder a suitable piece of cable to each bared wire and tape the joints.
5. Route the cables to the cavity exposed behing the left side bulkhead cover.
6. Solder the buzzer units to the cables from the turn signal unit plugs and tape the joints.
7. Check that the buzzers work with the turn signals.
8. Use pressure sensitive tape (or glue) to fix the buzzers in the cavity and secure the cables.
7. Re-fit the light units and covers.
If you find the buzzers too loud, put a piece of tape over to dampen them.
I hope some of you find this of interest - it is certainly a useful safety aid to know when your turn signals have not been switched off.
Good Luck -
PS - Noticed that when posted the sequence of the pictures reversed so please view them backwards - sorry!