How do i file suit of a car i just paid off airbags about to deploy

i purchased a car from drivetime 3-8-07 just paid the car off early 7-18-09 . i'm driving on the highway going aboout 75 mph and the airbag sensors are going crazy , pull the car to body shop and was told the airbag was about to deploy , i have written letters to the company and requested the finance charge back did not get and responce they charged me $21,900 for a 2005 ,car worth $8,300 i have since recanted on the last $8000 i owed and i have the title to this car , i want to go and trade it in or sell it but not sure if the company will try to give me a bad credit rating , WHAT SHOULD I DO , PLEASE HELP