It looks as if the informal ways of resolving this are over; your choices are suing and suing. Perhaps going to small claims court is the answer. I see that you are from Illinois--there could be claims that can be addressed by our Automotive Repair Act. If not, then breach of contract..
First, Dmitry is right. But here's the general rules that apply just about everywhere when you take your vehicWhen you take a car to a repair shop for them to do any kind of work on your car, you have the right to expect the work will be done right. And unless they use special language to avoid giving you a warranty, in most states you automatically get a warranty that they are going to do the work in a "good workmanlike manner." That basically requires that they do the repair or service work right. If they don't, then you have a right to recover damages for what they do wrong or the damage they cause (or, if they don’t return the car at all, for the value of the car itself). But you have to be able to prove that the reason for your new problems is something that the repair shop did or did not do. In other words, you have to be able to show that it was fault of the repair shop. But you first have to give the repair shop the chance to make it right so the first thing to do is ask the repair shop to fix it. If they won’t do it under their own warranty, then you have the right to get it fixed somewhere else and then hold the first repair shop liable for the repair cost. If the cost is less than a thousand dollars or so, you may be able to sue the shop in your local small claims court without needing a lawyer. Doing the repair work badly may also violate other consumer protection laws in your state. Because the law is different in each state you need to talk to a local Consumer Law attorney. And that brings us back to what Dmitry said. It's time to call your local attorney's Bar Association and ask for a referral to a Consumer Law attorney near you or you can go to this web site page for a Free Online 50 State National List of Consumer Law Lawyers (http://www.ohiolemonlaw.com/ocll-site/ocll-locate_local.shtml) and find one near you (lawyers don’t pay to get listed here and most of them are members of the only national association for Consumer Law lawyers, NACA.net). But act quickly because for every legal right you have, there is only a limited amount of time to actually file a lawsuit in court or your rights expire (it's called the statute of limitations), so don't waste your time getting to a Consumer Law attorney and finding out what your rights are.
This answer is for general purposes only and does not establish an attorney-client relationship. The law in your state may differ and your best answer will always come from a local attorney that you meet with privately. If you need a Consumer Law attorney, click the link above to find a Consumer Law attorney near you.
In addition to these other wise responses, it is not unusual for a state to have an investigation process into licensed folks who do not do a good job and in turn, that pressure, forcing them to do the right thing, so...
Consider contacting the proper state licensing authorities for this business to see if they will investigate your issues.