Check the flasher module at the fuse block and make sure it is properly inserted and not falling out. Virtually all 1990 Mustangs are missing the fuse block cover that keeps stuff in place due to poor engineering by Ford that caused the clips that hold the fuse block cover in place to become broken. If the fuses and flasher are OK, then the turn signal switch might be at fault provided the turn signal lamps are OK. Ford issued an ignition switch recall on 1990 Ford Mustangs due to faulty ignition switches that could short out and cause a fire. The engine might not crank if the ignition switch is failing. Also the neutral safety switch for automatic transmissions or clutch pedal switch for manual shift transmissions might be failed, keeping the starting motor from cranking.