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I've checked the wiring diagram, in viewing that and based upon your description, I'm going to say, you have a broken turn signal switch in the steering collum. The plastic housing breaks, and the contacts have become mis aligned, causing the malfunction.
I forgot to mention they checked the switch in the steering and it wasn't damaged.
Do you know if your car is equipped with "On-Star" capabilities?
Ok, let me examine the wire diagram and in a few moments I can give you an answer.