If the front turn indicators are both working then there is not fuze or turn signal switch problem. The dash indicators are driven off the same wire that the front indicators are wired to. The left dash indicator is powered by the Gray/Black wire.
As previously mentioned try cleaning the contacts for the bulb socket.
Also check where the wire harness connects to the dash panel. Possibly a dirty or corroded connector contact there.
Click on Figure 4 for the wiring diagram.