Take them to small claims court. Nothing to lose but a little time and a registration fee.
send an properly worded email with details of the issue
tell them you would like to settle amicably
go get estimates from dealership and another source (3 is best)
provide them the info in the email
Ask them to let you know what they want to do by x date in the hopes you can settle amongst.
Now if they fail to reply, file small claims court
you are ahead of the game now because you made an effort and honest ppl don't go to such lengths if they are lying.
Also they have to pay for rental car, in letter tell them you will find another mode of transportation while the car is being fixed and you are trying to help keep their costs down by not getting a rental car...you look REASONABLE
fix the car- file your case
dress properly, speak calmly and factually