If your tail light crack was caused by some impact, it may have also loosen the wires or cracked other connections behind your bulb socket.If rain water manage to get into the crack, there could be corrosion developing at the base or behind the bulb.Inspect and thoroughly clean out or buy a new bulb socket. You can use a multimeter to measure the ohms for continuity to ensure the stability of your wiring surrounding your turn signals.Any low resistance means their is continuity, 0 ohms or infinity means there's an open somewhere between the two points on your probes.Good luck!