It sounds like you have tried to get the repair done to your satisfaction but the shop disputes your view of their responsibility for areas around the windshield. You might consider filing a complaint with your state auto repair regulatory agency. You can use Legal.com to search for the agency in your state. Try search terms like [state] auto repair consumer protection, car repair law, etc. In California, it would be the Bureau of Auto Repair. Depending upon how your agency is run, you might be able to find guidance on what the shop should or should not have done, and what recourse is available to you now.
You can also sue the shop in small claims court for the cost of repairs. But you would have to be your own expert (or bring one with you as a witness), able to prove to the judge that the corrosion was caused by the windshield repair – in legal terms, that the shop was negligent.
There are legal plans that make attorneys available to consumers to answer questions specific to your case, provide guidance, review documents, write letters and other services. Click for a description of one such plan.
Hope this helps! — Dave