General Motors review: Service at Coughlin Chevrolet in Newark Ohio 43055

Coughlin called and set up an appointment to have the airbag replaced, they said they had one put back with are name on it. We leave over an hour from the dealership, so we drove up there and dropped off the Tahoe and drove back home to get a call to say they do not have the part after setting up an appointment. We had to drive back up and pick up the car. We had over four hours of road time and the car is still not repaired? The time before that I ask to have the front deferential plug removed and they said it was not made to be removed, I went home and did it myself to check the fluid. And the time before that I had my truck worked on for a rapid misfire and other work (over $3,000.00) to have the same thing the next day, Took it back and was told it needed a motor for another $8,000.00 after I already put $3,000.00 dollars in it (truck had only 115,000 miles on it). I ask can GM help?