My Contractor failed to obtain building permits to remodel my home now the work is going bad where do I stand?

Help, I had a contractor recently remodel my basement as well as other ares of my home, now the sewer they changed is backing up almost every couple of months, the plastering they did is cracking along with with other major and minor problems. About 3/4 of the way of the remodeling being completed a complaint to the buildings department by a terminated employee resulted in me finding out the contractor never obtained proper permits or consulted an architect to conduct the demolition for my home improvement. This contractor plays dumb when I tell him the plumbing they did is bad. They are avoiding the issue at hand by side stepping. The weird part of this ordeal is that the Sewer and Drain company they use to unclog my sewer also agreed this contractor made bad connections or their term a bad cross connection causing this sewer to frequently back up to the point the toilets over flow causing internal damage to the newly installed sheetrock that now has buckled. In an attempt to cover up their error the contractor stated he fired the salesman who set up the contract and who never got the proper building permits. I questioned the owner stating don't you review your newly obtained contracts before starting a new job making sure the proper doents are in order of course the abba dabba talk kicked in. What would happen if I reported this and to who should I report them to now that i'm having these reoccurring problems caused by this contractor. My home is my castle and I spent a lot of money and i'm tired of all of this grief. They come and fix some of the errors but states the house is setteling thats why there are cracks in the plastering and corking that was done due to a railroad that runs past my home. I say this is just shoddy work done for under a years time in my opinion. My contract is still under warranty with them what can I do. If someone can guide me in the right direction to get this matter resolved I would be grateful. Thank you all in advance.