The best way to resolve this is to meet up man to man and put down the final ultimate date for the car to be completed and handed over to you. Also to tell the mechanic that if the car is not ready by then, you are prepared to take away the car and not pay anything they claim they have done. List out the items you see now or get someone who knows to help you list down the things you see are missing or uncompleted and tell the mechanic about them. Tell them you will recheck the list again on the day when it is agreed to be ready. If the meeting breaks down, give them a few breathing days to think over and agree a date and time to come back and discuss again with you. If still no go, you will have to play the proper legal game by giving them a letter or notice telling them you will be taking your car back in X number of days with all original items back in place. You also have to mention in your letter you reserve the right to claim from them all losses, any missing part, etc.
I believe this is better than making grouses on the internet hoping they can hear your voice and your car can be ready.